There are no two ways about it: This dating app was built for threesomes.

London-based startup 3nder — the name can be pronounced “Thrinder” to rhyme with “Tinder,” according to its founder — said it has raised $500,000 in seed capital from a pair of unnamed angel investors.

Since 3nder’s mid-2014 launch, nearly 1 million users have downloaded the iOS app in search of kinky hookups — whether “straight, bisexual, gay, poly- or pansexual,” says founder and CEO Dimo Trifonov.

The torrid growth has spilled into the US, where New York is 3nder’s second-biggest market behind California. Worldwide, users are sending about 1.2 million messages a month on 3nder while logging

around 4 million swipes.

That falls sorely short of the 1 billion swipes per day seen on Tinder, the hookup app whose viral growth is boosting billionaire Barry Diller’s IAC/Interactive.

Still, 3nder’s smallish subscriber base is a motivated one that’s willing to pay its monthly fee of nearly $13. Revenue is up 500 percent this year, and “it’s all organic, we haven’t done any marketing or advertising,” Trifonov told The Post.

Nearly a third of the accounts created on 3nder are couples, and most are millennials, according to Trifonov.

Indeed, old-school swingers have balked at 3nder’s Facebook log-in requirement, which is intended to verify identities.

Despite 3nder’s privacy guarantees, “People were saying, ‘I can’t log in with Facebook — I’m gonna lose my job,’ ” according to Trifonov. “But people between 18 and 34 are not so affected by imaginary moral values.”

The soft-spoken, 25-year-old techie admits his own tastes run vanilla. He got the idea when his longtime girlfriend confessed an attraction to a woman.