The answer depends on a few things. How capable are the two smaller subs. From personal listening experience, the 175 just does not reach low enough to compete with the 500. The 350 does.

The other is the characteristics of your room. In may case I get very little difference in response from my sub regardless of where it is places. I believe this to be because I have what turns out to be a square room at the subsonic level. My case is unusual though.

I vote for the ep500. Two 350s would be another good option

Definitely 1 EP500. I've tried an EP175, and currently have an EP350. If I upgraded, I'd go for a single EP500 over 2 EP350.