Three angels are OK, number-wise, for a 55 gallon tank. But if two of them pair up and begin to breed, they'll probably harass the third angel, which wouldn't be good. I'd recommend only 2 angels per one tank - be it a 30 or a 55, or even a 75, gallon tank. This way, whether they pair up or not, there is no "third fish left out and bullied". I suppose one could have 4 (or 6) angels in a 75 gallon tank, and even more angels in larger tanks because in such tanks there is more space to run away and hide if any fish are bullied. But nevertheless, angelfish can be very territorial and quite aggressive during breeding times, and for this reason it's best to have a mated pair (or just 2 angels of the same sex) per one tank. Unless, you were 100% sure that all of the 3 angels are either males or females. And this is not possible - you can only know their sex once they start to breed. If you get 3 angels, there are 4 possible combinations you'll have: (1) 2 males and 1 female; (2) 2 females and 1 male; (3) 3 males; and (4) 3 females. Therefore, there is a 50-50 chance that you either will or will not get a mated pair. And a mated pair - as mentioned above - may bully the third angel.
That's my opinion
