I am currently using two 8800 GTS 640MB GPU's running on SLI. Would there be any sort of gain running a third GPU when playing Counter-Strike: Source or World of Warcraft? Opinions, please.

I doubt you'd see a gain from a third.  I'm running CS:S on an 8800GTX, settings maxed at 1920x1200 and I get like, 220fps.  I know a GTS isn't as uber as a GTX, but it shouldn't be too far from the same results.