Before everyone goes all??? Omg that's so unexpected, let me tell you that these subunits have actually been pushed for a long time by Bighit
1. First appearance : Webtoon

2nd Appearance : Pairings in Persona. Every version has a pairing of the characters of BU, for example, 1 will have Jihope, 2 Vmon, 3 Yoonkook and 4 Jin. These are indicated by the notes. There is whole thread by @cafe_army that I can link later if you wanna read more.

So basically they paired up Vmin, Namgi and Hopekook for Persona series ( other notable pairings were BST pairings in Tear)

3rd Appearance : BTS WORLD
The characters in BTS world are linked in Chapter 6 and appear in each other's last chapter. Guess what the pairings are again? That's right. Hopekook, Vmin and Namgi.

4th appearance : Festa photo shoots

Now look at the 3 subunits
Jamais Vu : Hopekook
Respect : Namgi
Friends : Vmin

Just in case anyone reads this thread, here is the theory by @cafe_army on the album pairs https://threadreaderapp.com/thread/1095986453308272640.html