Yes, even though the third slot will run in 8x mode, it will be more than enough for a 3rd card. Whether you will see much of an improvement will depend on the drivers. What resolution are you running at?
lol you hungover today? he clearly asked about adding a single 760 to his mars 760 so that would be tri sli.The Mars 760 is a dual GPU card, isn't it? I'd say it depends on the mobo and whether it supports quad SLI. I am confused when the OP says Tri-SLI with that in consideration. Does that mean 3 mars cards? That clearly wouldn't work. Or two Mars cards? I dunno.
AFAIK nVidia doesn't allow SLI between dual and single GPU cards.