Are you using Mainstage 2 in 64-bit mode? This will enable you to take advantage of more than 4GB of ram.
Omnisphere stores and reads the sample/synth library data in RAM... so this will help.
However, one question: Are you running 3 instances of Omnisphere each with a single patch... are are you running 3 instances of Omni, each with upto 8 patches?
If the former, then better to use one instance and configure Omnisphere as a multi-instrument.
If the later... then I'm not surprised... that's quite a strain you're putting on your Mac! Omnisphere is amazing but it is pretty hungry.
So, rather than have 3 separate instances of Omni, each with one "sound" or patch each, it's better to load One Omni as a multi and switch between the 3 patches using one instance.
Hope that makes sense.