I agree, the snap toggles with a stud should be fine. If you have a mechanical engineer as a friend, you can confirm with him. But, yeah, I was gonna mention this as well. HD has several types of anchoring mechanisms for attaching heavy weight to drywall.
The other option I like, is attaching a 1 by or 2by across your three studs, and mount to that. That should still be hidden behind a 55" TV.
Home System: NAD 747 AV Receiver - Rotel 6 channel amp for other zones - Ascend Acoustics CMT340SE mains on pedestals - DIY Sub with 12" Shiva! - DIY center very close timbre to CMTs - PSB Alpha rears - Yamaha NSAW-570 outdoor speakers - Audio Engine B1..Love this - DVD/CD...need an upgrade
Ranch System: Pioneer VSX-531 - Pioneer AJ SPBS22 bookshelf - Polk PSW10 sub