Like many students, the shy boy who studied computer science got drunk in the college bar with a girl from the year below. They snogged and – sharing a love of photography, computers and cups of tea – fell in love. Six years later, they married. A few years on, however, and this everyday story turned in an unexpected direction when the young man's hair began to thin.

"That was the point I was no longer able to be in denial – time was catching up, testosterone was catching up. I had visions of myself as an old man sitting in a nursing home waiting to die, crying all the time and nobody understanding why. I thought, this is intolerable, it cannot go on – I've got to do something."

The young man became Sarah, now a chatty, self-assured city councillor who lives in Cambridge. A stereotypical way of describing trans women in childhood is to say they feel like "a girl trapped in a boy's body," says Sarah, but she believes few people think at that level. "As a kid, I assumed that everybody wanted to be a girl and some people were lucky enough to be born that way. Then it very rapidly became clear that this was something that we did not talk about and it got buried very, very deep inside."

As a teenager, when she walked past window displays of women's clothes, she would avert her eyes "in case someone saw me looking and would know. That's the level of paranoia and it took a long time to get over it."

Only a small minority of marriages survive one partner changing sex and Sarah is no longer married to Sylvia, the woman she fell in love with when they were students. But this is only because the law forbids two women to be married. When Sarah completed her transition with gender-reassignment surgery in January 2007, she and Sylvia were compelled to divorce, despite the fact that they were very much together. So they are now civil partners instead, with a wedding album and a divorce certificate stowed away in a drawer in their house.

Sylvia, who works in computing in Cambridge, is remarkably phlegmatic about her partner's change of sex, and describes herself as "heterosexual by default" before Sarah's transition. "I never really considered dating women before, but when I look back, the relationship Sarah and I had when she was presenting male was a bit lesbian," she says. "The dynamics were a lot more like two women living together when compared with other opposite-sex relationships. I had no objections when Sarah wanted to transition. I knew it would be difficult and I worried about whether both of us would experience discrimination and whether it would be scary, but I didn't see a problem adapting to the fact that someone I loved was going to have differently arranged genitals. No biggie."

Sylvia even had some laser and electrolysis treatment to remove her armpit hair so she could empathise with some of the physical pain that Sarah was experiencing.

After Sarah's surgery, it was not simply a case of Sylvia loving the person she had always loved. Like any big life change, transition affected Sarah's personality. "Before she transitioned to female, she was really quiet and nerdy and I was doing the talking for two," says Sylvia.

As a woman, Sarah is now forthright and confident. "It's amazing how people blossom and evolve when their relationships change," says Sylvia of Sarah. "There was something really nice that was brought out when she transitioned."

For Sylvia, the toughest part of Sarah's transition was being forced to replace their marriage with a civil partnership. "I thought it wouldn't make a difference," says Sylvia. "I'm a scientist, I'm rational. It's just a bit of paper, but it made us cry." In contrast to the poetry of the wedding vows, they found the language of the civil partnership ceremony like a business arrangement.

Sylvia and Sarah hope to remarry when the marriage (same-sex couples) bill becomes law, but their original marriage can never be restored in the eyes of the law. "When the registrar pronounced us civil partners it felt like the state was kicking us in the teeth," adds Sarah.

The transformation of their relationship did not end there, however. When Sarah was transitioning, she struck up conversation online with Zoe O'Connell, a computer network manager who is also in the Territorial Army. Zoe was seeking a good place for laser hair removal in East Anglia and Sarah was able to recommend one, so they met for a cup of tea. Bonding over their shared experience of transition, they became good friends.

While Sarah's path to gender reassignment surgery had been gradual, Zoe had a lightbulb moment. Like Sarah, she had entered into a heterosexual marriage; unlike Sarah, Zoe had three children with her wife. And it was not until the marriage broke down eight years ago that she began to question her gender.

"Splitting up, I tried to prove that I was a bloke and failed spectacularly. I had my hair cut short and joined the army," Zoe remembers. When she saw a photograph of herself with a shaven head she realised: "That's so not me."

She began to explore trans issues. "I was chatting to someone who I knew was trans online and it was a sudden dawning – 'Oh right, this isn't normal, I'm a girl. Shit.'"

When Sarah had surgery at a hospital in Brighton, Zoe accompanied Sylvia and they fretted in a pub. Zoe spent a period "part-time" before going "full-time" four months after Sarah. "It got ridiculous," remembers Zoe. "At one point I ended up flip-flopping between boy-mode and girl-mode seven times in a day." Zoe was treated at the same hospital as Sarah.

Some months later, Sarah and Zoe went to Brighton again to support a mutual friend's surgery and this time shared a twin-bedded hotel room. "There was sexual tension in the room," remembers Sarah, laughing. Sarah and Zoe were falling in love.

Feeling increasingly stressed about their feelings, Sarah, Zoe and Sylvia sat down to talk and, together, they "renegotiated the bounds of the existing relationship," as Sarah puts it.

Soon afterwards, Zoe moved into Sarah and Sylvia's house. At first, they tried sleeping together in a big bed but the person in the middle was always very uncomfortable. Now Zoe has her own room and often sleeps there, although the three all move between bedrooms.

Many people might assume that Sylvia would struggle to cope with her husband becoming her same-sex partner. For her to then accept the transformation of a monogamous relationship into a polyamorous one must be an enormous strain. But the striking thing when I meet Sylvia, Sarah and Zoe at their home is the absence of strain: their unconventional domestic arrangements – they also have five snakes – soon seem completely normal, perhaps because they are all so at ease with each other.

"A lot of people looking at this from the outside would probably see you as the long-suffering wife," says Sarah to Sylvia.

"None of the gender or poly stuff has ever been a problem," replies Sylvia. "Irritating personal habits are far worse."

Sylvia admits she felt "a little bit excluded" when Sarah and Zoe were going through transition – "I could never quite know what it was like" – but this period also helped her bond with Zoe: they both worried about Sarah's operation.

What about polyamory, though? "We had a lot to gain and nothing to lose really," says Sylvia. "Transition made my relationship with Sarah that much stronger because we'd been through so much together, so it was like, well, we can do anything after this."

Sarah, Sylvia and Zoe are not "poly-evangelists" but simply say it works for them. "Some people are quite jealous or need that level of commitment from one person that you can't get in a poly relationship so it doesn't suit everyone," says Zoe.

Isn't three fundamentally an awkward number? "It can be really handy," says Sylvia. "If two of us are massively disagreeing about whether to do something around the house, we can have someone to break the deadlock. As long as you're careful enough so it doesn't end up with two people picking on one."

"We're all adult enough not to do that," adds Sarah.

Their relationship is a "triangle" (a "V-shaped" polyamorous relationship in which not all three members of the relationship are connected would be more tricky, they say), and they share many passions. Zoe and Sylvia go geocaching together; Sarah and Zoe go horseriding. All three women have Lib Dem politics (Zoe hopes to become the first openly trans person to stand for parliament for a major political party) and computing in common. They also enjoy climbing, mountaineering and canyoning together. With Zoe already having three children, they are not planning any more. "I'm not into children," says Sylvia decisively.

"I've got three already," adds Zoe. And Sarah? "Governed by the other two really," she laughs.

Zoe maintains a good relationship with her ex-wife and sees their two daughters and son every weekend; she, Sarah and Sylvia usually stay with them at Zoe's parents house. As children tend to be, hers are very accepting (they don't call her Mum or Dad – simply Zoe) although Zoe admits to feeling bad if "they get some stick" at school. "They were too young to understand gender when I transitioned and when the three of us got together they just mentally shrugged," she says. "It is normal to them."

Zoe has had no problems in the Territorial Army either, but it has been harder to gain acceptance from older family members. While Zoe has an excellent relationship with her parents and her brother, Sarah has been estranged from her father since her transition, although her relationship with her mother is "better than it's ever been". Sylvia's mother is "accepting", but her grandparents struggled to adapt to her husband becoming her female civil partner, although Sylvia and Sarah were reconciled with them before they died.

Living in Cambridge, they rarely experience discrimination or abuse. Sarah is not the city's first openly trans councillor, and when they pop down to their local real-ale pub: "No one bats an eyelid. We're probably some of the less 'out there' people," says Zoe.

They do not hide their living arrangements but Zoe says they invariably develop a language of "partners" and other careful phrasing to avoid having to constantly explain themselves. "If your living arrangements or social situation is abnormal, you don't just come out once, you come out all the time," she says. But occasionally it can be fun to observe people's reactions. "I come out as trans first and then bisexual," says Zoe. "Just when people are getting really confused you hit them with 'poly' and their mind just explodes."

All three women feel liberated by their different experiences of transition and they know quite a few trans people now living polyamorous lives. "Gender transition is one of the most sexually taboo things you can do, and you do it and you realise the world doesn't end. Then you start thinking, what other things have I always taken for granted that are just wrong?" says Sarah. "In some ways I resent being born trans because it's been a lot of pain, a lot of hassle, and it has dominated my life. But at other times I almost feel grateful because it has given me an attitude that almost nothing is sacred and I don't have to be a prisoner to this very English 'mustn't make a fuss, mustn't challenge things' life of quiet desperation."