Threesomes

No, not that kind of threesome - get your mind out of the gutter!  I'm referring to a group of three best friends.  I have been in situations in which they can become too difficult to manage due to two of the three becoming close and/or hanging out without one of you present. 

Sometimes I wonder if it's possible to have two best friends, who are also best friends with each other.  The three of you can't always hang out, and so it will naturally happen that one of you cannot make a dinner or be available to take a phone call.  Is it possible to keep up a threesome?

Maybe I'm not able to be in this type of situation because I find it gets too complicated.  When you have one best friend and spend time with her, it provides time and space for the two of you to grow.  If there is another person involved, I find it can get somewhat chaotic.

I experienced this a few years ago.  I thought I was good friends with both women, and then came to find out that they had planned a get-together without me - one that involved one of the women bringing her spouse to meet the other friend.  I was upset since we had always planned things together, although now I realize that I over-reacted and that these things happen.  The two women were good friends too, and are/were allowed to spend time together.  But it still feels crummy when you feel left out and may not understand why.

Have you experienced this?  If so, how do you manage the threesome?