Three's A Crowd

So the roommates want to go on vacation, and they've said that other people are welcome to go. GuyRoommate even said he thought it would be a good idea if I went, I've been pretty stressed about work, and a break would be a good thing. I just don't think I want to go. Wait, that's not exactly it...I do want to go on vacation, but there's a reason I don't want to go. If it's the two of them, then I'm the third. They've talked about inviting other people, but it's another couple, so I'd be number 5. Yet again, odd man out. Most times being single doesn't bother me, but at times like this is kind of does. GuyRoommate has said repeatedly it wouldn't bother them to have me there, I'm just a part of the group, not an extra, but I feel self-conscience about it. Sometimes if it's the 3 of us out to dinner the waitstaff asks how to divide up the check. "Oh, so it's you two together and you all alone, by yourself?" What makes it even worse, I don't even feel like I have a friend that I could invite. Booo!