This is a very difficult question to answer precisely, but as of now one 780 ti will run most games on max settings at well over 100 fps, which suggests that 3 would probably yield 250 fps+ (assuming you had a cpu that could keep up, which would be difficult if not impossible to find). If we assume that by "support gaming on max settings" you mean a single 1080p monitor at a minimum of 60 fps in the most demanding games, I would imagine that that would be possible for at least 5 years, and probably closer to 7 or 8, perhaps even more.