What size is the tank? Is it planted? How long have you had the angels? Are two of the angels staying together?
What could be happening, depending on the age of the fish, is that two have paired off. This leaves the single Angel which they will try to drive out of their territory. If the tank is smaller the single has no where to go and they could possibly stress him/her to death. Adding other angels would depend on the size of the tank. Another thing you can try is remove the two angels leaving only the picked on angel, rearrange your tank so the line of sight is broken from one end to the other, then readd the two angels. What this does is trick the removed angels into thinking they are in a new tank which means they have no territory and often lowers or eliminates aggression. Rehoming the odd angel is also a very good option.