Does two GTX 760s SLI have the same power as a GTX 770? If I add three 760s will it have the same performance as two 770s?
It will be significantly more powerful than the 770, more in-line with the performance of a 780 Ti. The problem is, it can be inconsistent because games aren't always well-coded enough to take advantage of two cards, let alone three. It's generally recommended to stick with one good card you like instead of two ok ones.
Two 760s will outperform a single 770 but may limit your future upgrade options. I would never suggest getting more than two video cards. Three and quad SLI are really inefficient.
Don't bother with SLI at all if you can help it. Just get the best single video card you can afford.