Personally I'm not a big fan of 55's, they aren't very wide (13" outside measurement usually). That leaves just a little over 12" of turning space and with large fish that's not very much. But in answer to the question 3 is likely the most apropriate number, any more than that and you are overcrowding them which can induce stress and cause fights, which in turn could lead to one or more deaths.
I cant agree more.. when I had 4 in my old 55 gal, 3 had established territories which had the 4th one running around and hiding just to find a spot to post up in. They all shoaled and ate together when feeding time came, but it was too small and no balance in the tank.