Hello and welcome to Psych Central! Sometimes people feel better in a threesome than a twosome, depending on what they want to do and how they feel. For example, two of them might like bars and the other one doesn't, so they don't invite the third one. Just two people together calls for more intensity.

I can't say why one doesn't invite you when the other one is not around. But she might feel closer to the other friend than to you.

How about getting out a bit to see if you can make some other friends, too? Maybe someone who has a common interest, is taking another class with you, in a social club, etc. Then what these two do won't be as bad a concern for you.

I am leaning toward suggesting that you not bring it up to them.

But let's see if other folks have some thoughts on the topic.