Upto 3 doesn't hurt my dear Sir. People do Triple CFX on their system to play games and they don't even have the option of providing extra power through risers since gamers can't use risers. I'm quite sure 3 is in the safe zone.
If its a high end mobo which is crossfire compatible etc, then yes. Doing it on cheap boards is a gamble.
Oh right! Well I've run 3 cards on a gigabyte mobo costing 7k, though not too long two Weeks. How cheap are you talking about?