I'm the hinge of a vee, with two men who are very good friends. I was with FBF for about two years, broke up with him, started dating CBF, have been with him for about 7 years now. Last December, FBF and I found out we still wanted to be together, and CBF was all for it. [<- reader's digest condensed version]
FBF is intensely introverted, and private, and loves to be alone most of the time. CBF is very likely ADHD. He rarely remembers when I tell him that I'm going to see FBF) who is moderately long distance. So he often calls. FBF takes this with quite a bit of grace. When I went to Vegas with FBF, we had a phone call where we both talked with CBF, directly and from the background. It was a long, cheerful call; reminding of times when we've all hung out together in the past. Three of us have not been together since we've started this poly vee thing.
When I'm with one, I almost always think of the other. A lot because I'm in love with both of them; and a lot because I'm so happy they are both okay with this. (or they seem to be...)
Recently I had the first time I was with CBF, at his mom's for dinner, and FBF called. (Mom knows the whole history, but not about the vee)(I'm fairly certain she doesn't)
It was a little nerve-wracking for me, but nobody seemed to notice or care, and nobody has had any trouble about it since.
I relate to your challenge in feeling forgotten, daily. Mostly, I try to distract myself with my life. I try to remember they wouldn't be with me if they didn't want to (these men don't do much that they don't want to).