This was actually inspired by how my oc Jessie got together with his bf, another oc, in my chatfic 'We love Balls' (Shameless plug-) but changed up a bit. Ships (and the third wheels) in mind are:
-Asanoya (+Tanaka)
-Ennotana(+Narita/Kinoshita)
-Arankita(+Omimi/Akagi)
-Osasuna(+Atsumu)
-Semishira(+Tendou/Goshiki)
-Ushiten(+Semi)
-Aofuta(+Moniwa)
-Kyouhaba(+Watari)
-Iwaoi(+Hanamaki/Matsukawa)

...and a lot more honestly, but here are my favs <3

Okay, so one one of them gets letters in thier locker from a secrect admirer. They tell the other one, not knowing they are the ones writing it. After a few letters, some random takes credit for the letters and the 2 go out. It leaves the other heartbroken but makes no move to tell the truth as they want them to be happy. The third wheel, who knew about this in the beginning decides to fix this and leaves another letter in the locker. It says something along the lines of 'I knew who is writing these letter's. They are my friend and I refuse to let someone take credit and let them suffer. From, a friend of a lovesick fool'. They decided to confront the 'letter writer' and find out they lied. Soon afterwards, they tell the true letter writer, and at next practice they confront the third wheel about it as they were the only person they told. By chance, the person who got a letters walked in and realized what was going on. The third wheel got the f out and let the two talk about their feelings.

Bonus: The third wheel comes back as they are kissing 'Are you too gonna practice or you gonna suck face?'