Can a non-retina iMac with i7 3.5 Ghz and GTX775M handle three displays?
I am assuming that you are referencing a 27" Late 2013 iMac because there is no 21" iMac that has that specific CPU frequency.

You can have up to two external displays connected via Thunderbolt  according to the specs posted on EveryMac.com



I have used 30" and 43" displays with my iMac and there has been zero lag.  I also only have SSDs (no hard drives) so I have had zero issues with heat, even with multiple displays.