Remove the 3rd angelfish. Its trying to break up the pairing the other 2 have formed. Either the pair will fight back and eventually kill off the 3rd fish or the 3rd fish will kill its rival and take its place in the pairing.
How big a tank do you have?
I had the same issue, despite trying to manage the situation for a year i eventually returned the 3rd fish to the store (got a little bit more back for it than i paid). Something would have died eventually and i have a much less stressful tank now.