I've done probably 10 systems with 2 antennas (can't ever remember doing 3) and they've always been at least 50' apart. I've never had the balls to try them closer.

So far, I've never experienced they conflict issues that have been reported by having the antennas too close.

I've also done systems with 2 RFXs and 2-3 MSC400s and, by and large, they work well (sometimes suffer from the ports opening up and command collisions when multiple people are using multiple remotes at the same time)