What CPU you on at the moment?
Single player is GPU bound, multiplayer (especially 64 player servers and full) will crush a CPU not up to the task.
Especially since you have tri-SLI, as that's very tough on a CPU since so many bits are shuffling around like mad.
If it's an Intel and recent model, try disabling HyperThreading. It could help boost GPU utilization. I had to just to make sure I didn't see any weird motion issues... it wasn't microstutter but it was a very weird and related issue - things in motion speed up and slow down repeatedly, doesn't feel like stuttering as it looked smooth, but had a weird visual "feeling" going on. No HT fixed that up, and I've heard of disabling HT helps with utilization of multi-GPU systems in some cases.
My 2600K @ 4.8ghz happily complies with 64p MP, complete with all the explosions and chaos displayed in almost maxed Ultra settings (no MSAA, dropped World Mesh down to High) with 2x 560 Ti 2GB cards. Rarely dips below 60fps (v-sync).