Three years ago, when Chelsea Blakeburn, 23, told her then-boyfriend that she was sexually fluid, he jumped with glee.

“He asked me if I wanted us to do a three-way with a girl,” Blakeburn, who lives in Hell’s Kitchen, tells The Post. “Straight guys love it when girls say they’re sexually fluid.”

Unlike bisexuality — which is a sexual and romantic attraction to both men and women — “sexually fluid” is a term coined by millennials who say that their attraction for males, females and every gender option in between can fluctuate over time.

“I think ‘sexually fluid’ is a more appropriate term [than ‘bisexual’] because we’re all attracted to different genders in varying degrees,” says Blakeburn, a Ph.D. student at Columbia University, who believes that all people exist on a sexual-fluidity spectrum. “Why lock ourselves into such a small box?”

She isn’t the only one embracing the new title. In June, TV’s “Younger” star Nico Tortorella revealed that he identifies as sexually fluid. “I don’t see the world as gender binary — man, woman. There’s a whole spectrum of people that fit into all of it, and I’ve dated all of them, a lot of them,” he said on “The Drop-In With Will Malnati” podcast. Other celebs who publicly identify with the label include Miley Cyrus, Lily-Rose Depp and model Nyle DiMarco. In fact, a 2015 study by the UK’s YouGov found that 43 percent of 18- to 24-year-olds identify as sexually fluid.

Luke Davis, a 25-year-old student at Parsons, also identifies as sexually fluid. While he’s dated both men and women, Davis says he’s currently more attracted to men.

“My sexuality is on a spectrum,” says Davis, who lives in Inwood. “I’ve had some deep emotional bonds with women, but right now I’m more physically attracted to men. My attraction and orientation change over time depending on where I am in my life. It doesn’t mean that I can’t make up my mind … At the end of the day, everyone is free to label themselves as gay or straight, but I think we’re all sexually fluid.”

Like most young people who are coming to terms with their sexuality, 18-year-old Emir Brown has faced identity struggles.

“When I was younger, I identified as straight but suppressed my attraction to men because I grew up in a conservative household,” says Brown, a rising sophomore at Atlanta’s Emory University from West Harlem. “But after being around queer people at school, I realized it was OK to like whoever you want.”

All three say that their families haven’t yet come around to embracing their sexuality. Blakeburn says her conservative, Methodist parents don’t understand her new identification.

“I came out to my mom, and she just laughed it off and said that it was just a phase,” she says. “She thinks I’m just experimenting and will eventually find the right guy, but I know who I am and who I like.”

Blakeburn says some lesbians are also wary of the label.

“I’ve gone on Tinder to specifically try to meet women, but some lesbians say that I’m actually straight and that I’m just wishy-washy,” she says.

Blakeburn and Davis both say they’ve faced scrutiny from some members of the LGBTQ community because of their identity.

“I understand where they’re coming from,” Davis says. “Gays and lesbians have fought so long to claim their identities, and some can see sexual fluidity as a threat to them.”

But experts say that “sexually fluid” is a valid label, and that it even harks back to ancient cultures.

“In some ancient Native American cultures, there was no such thing as gender,” dating expert Sameera Sullivan, founder of matchmaking service Lasting Connections, tells The Post. “I think a lot of the time society tells us who we can and can’t love, but I guarantee you if we didn’t identify as male and female, we’d all be sexually fluid.”

Blakeburn hopes that the recent coming-out stories by celebrities will open up dialogue about sexuality.

“Before, I’d tell people I was bisexual because people would have less questions,” Blakeburn says. “But now I tell people I’m sexually fluid because it’s more reflective of who I am. I hope [these celebrities] are able to raise more awareness.”