When I was in a FMF triad, I worried about this a lot. I finally told them that I worried about how much time I spent snuggling with each of them because I worried the other would feel left out. We resolved it to an extent by just being aware of the issue. They told me to snuggle as I saw fit and they would try to do likewise. I still worried, but I don't think it was as big of an issue as I was making it.
I think the bigger problem was that I saw this in so many of the things we did. Who sits in the front seat in the car? On a rollar coaster ride, who rides with whom? Those are the things that society has pretty good rules of thumbs for monogamous couples, but nothing really for polyamorous people. The best I could do was to try to take turns and keep track. (But I think that was probably not the best long term solution.)