It seems ati tray tools can only support 2 cards, msi afterburner can do 3.  However, I think you may be able to get around this problem by connecting the two with a crossfire bridge.  I read that being suggested some where, I haven't tried it myself.  I just took one of the cards out and put it in another machine.  Linux can do 4 cards, but I can't seem to get around 'Configurable Peak Range'.
I also read that crossfire might slow it down a little.  I had a few cards in crossfire not long ago, I am not sure if they really mined any slower.