Maintaining a long-term relationship can be difficult, but Romy Madley Croft, Oliver Sim and Jamie Smith have managed to do it – and they've become megastars in the process. They make up the band the xx, and they've been making music together since they were kids.

The band's two singers, Croft and Sim, met in preschool in a London suburb when they were only 3 years old. "They had music classes, and I remember sharing a xylophone in that music class," Sim says. "I've also got pictures of it, so I'm sure I'm building memories off a picture."

Croft says another old photo of the two of them sticks out to her. "There's one of us standing on a bench as if we're sort of on a stage. Oliver's on the side that he stands on [when we perform] and I'm on the side that I stand on, which is quite a beautiful coincidence, I think," Croft says.

Their friendship with the third member of the xx, producer Jamie Smith, began a bit later, when the trio was 11 years old. The three formed a tight-knit and insular crew.

"I was starting out a new school, and Romy and Oliver had come to a new school as well," Smith says. "There were only a few people there that I liked, so I was quite happy just to stay us. I think just keeping your closest friends around is the best."

The trio released its debut album, xx, in 2009, and have sold millions of records since then. All three bandmates have grown in different ways: Smith released a solo album under the name Jamie xx; Sim gave up drinking; Croft got engaged. Now, the group has a new album called I See You coming out Jan. 13.

Croft says one song on the new record, called "Test Me," touches on the challenges of close friendships. She says the lyrics describe "a sort of hard time in our friendship, between Oliver and I – and a time when all three of us were quite distant from each other emotionally and geographically."

But Croft says those challenges led to some important changes. "It sort of represents a new time of us actually talking about things rather than just pushing them down, and I think that was a good thing," Croft says. "Sometimes we would say things to each other in the music before we could say it to each other."

Croft, Sim and Smith shared these and other stories with NPR's Ari Shapiro. Hear their full conversation at the audio link.