Is it normal for friends to hang out ALL of the time?

So i'm basically the third wheel of the group. But anyways. my two bff's seem like they can't get enough of each other! Everyday friend1 gets off work she'll go home get herself together then go to friend2's spot. and just hang out until whenever. Every day, every weekend, every time, they're together at some point..but not me, either i'm working, or not invited except when they want something from me (thats how I see it). We're all 21 and this isn't just a summer thing this is an all season thing. Its funny though bc when friend1 has a boyfriend then that changes. But when that happens again (hes in jail and gets out in a couple months)....I don't want to be the rebound friend for her I just would like to politely say F you I was here the whole time before!