To answer your question... I am kind of going through this. I usually keep my distance when they are all lovey dovey and affectionate (when they are at that stage) and hang out with other people or chill by myself for a little. If you are feeling like that much of a third wheel that it starts to affect your friendships with them, I would hang out with them a little less. The last thing you want is to start to feel resentment or bitter towards your friends because they are in a relationship and you are not. And don't dwell on the fact that they are together and you don't have a bf/gf... that will only make you feel lonelier. Hope this helps. Stay strong and stay blessed!