Check out the happy threesome!

Jailey pal Kendall Jenner joined Mr. and Mrs. Bieber at the recording studio Tuesday night, dressed in head-to-toe Yeezy -- sweats and slides -- hanging with her fellow supermodel friend and her old pal Justin.

We spotted Kendall arriving at 8pm and the whole group left around 10pm.

It's good to see Hailey and Justin back together, after spending time apart while she was in New York working. And it's probably healthy for them to have a mutual friend around rather than being isolated in their marriage -- too much one-on-one time isn't always a good thing ...