I am friendly with two women that I met in college. Let's call them Anna and Rebecca. Because we all live in different countries, we only see each other every few years. I've felt for a long time that the two of them have a special dynamic together. Specifically, when the three of us are together, I often feel left out. Because of that, I haven't proposed trips with just the three of us for a long time.
Anyway, recently I went to their countries. I had originally planned to visit Anna first and then to go see Rebecca. However, Anna suggested she join me to visit Rebecca, and that's what happened.
Predictably, I felt left out. More specifically, I felt like Anna was dominating the visit and I could only watch. For example, Rebecca now has two young children and Anna got along much better with them than I did, especially since she knows quite a bit of their native language, while I only know a few words. She also has more experience with children than I do. I decided I wouldn't try to compete with her because I knew that would just make me feel worse. As a result, I felt like a third wheel.
On the one hand, I would like to bring this up, since I'm tired of feeling left out and uncomfortable. However, I fear that if I do this, they would likely get angry and the friendship will be over. We've known each other for almost 20 years, so I would hate for my friendship with these women to end. I get along fine with them individually.
When it comes to friendships, threesomes often tend to get complicated---regardless of age. As you have experienced, the relationship you have as a group is different than the one you have with Rebecca or Anna, independently. Because a third-person always adds another dimension to the mix, your story is more common than you might think.
It sounds like Anna has a more dominant personality than you (and probably than Rebecca) so she tends to become the de facto leader of the group. Your observation that she and Rebecca may be closer to each other than either one is to you is probably accurate; you sound pretty insightful. That doesn't really matter though; you probably have other friends to whom you feel closer, too.
You seem to value your friendship with both these long-time friends. Since you only get to see them every few years in small doses, my advice would be to just accept them and your threesome relationship as it is when you're together as a threesome---you won't be able to change Anna and you probably can't change your personality either. Don't consider yourself a third wheel; they are as eager to see you, as you are to see them even if getting together isn't totally comfortable.
It sounds like you may have felt a bit railroaded into the last joint visit. To avoid this happening next time, you could space each visit apart. If the two visits have to be successive (because of economic considerations, for example, airfare is cheaper to make both trips at once or you only have a limited amount of time off), explain to Anna, in advance, that you would like to visit individually this time because you see them each so infrequently.
When the visit after that comes around, which sounds like 4-6 years from now, you can revisit the decision.
Hope this helps.
Some prior posts on The Friendship Blog aboutfriendship threesomes: