This is complicated, I've personally never gone through this but from a relationship perspective. They will do things only between them since they are together as a couple. I understand that you feel left out but think about it like this. They're your best friends, they still want to hangout with you and see you but they also want to nurture their relationship. Put yourself in their shoes, if you were in a relationship with someone you really like, wouldn't you want to spend time with them? Wouldn't you want some time alone with them? Don't take it personal but think about the fact that they are a couple now and it's normal for them to spend time with each other and not invite you along. Maybe you'll understand their point of view later on but for now, I would say this is not worth you confronting them and ruining your friendship with them. Be there for them and I'm sure they'll be there for you too. If they're not around, do things for you, meet other people, etc. Like I said, it will be tough because one feels like a third wheel but eventually you'll come across new people too and maybe you won't be as close but they'll still be your friends. If you truly are concerned then express this to them but don't ruin your friendship.