I was in a long term triad MFF for over two decades that ended due to death. We were a vee. Now, I'll have to give our husband credit, he worked very hard over the fairness issue ~ sometimes to the point that it drove us both nuts. I do think that is very important for the first few years or when there is a new major change in life that disrupts routine. But after awhile, as you said, you out-grow the rules.
It could be that you have indeed reached the phase where you do not all have to be together all the time. Your mates are so incredibly sweet to want to include you and to so strongly look out for your feelings. If you want to go this next step, you need to reassure them and really mean it. If you can't really mean it, then you aren't ready. And part of the reassurance does mean making new rules so everyone knows what the expectation is without having to guess.