My ducks are fighting! helpp !?

i have 2 older ducks and bought a new one right before easter... the one i bought for easter is now old enough + big enough to go outside... the other two ducks HATE her and attack her What should i do? I was thinking about getting another baby duck and raising it wih the one i got before easter... but im not sure... any suggestions? SERIOUS ANSWERS ONLY ! thanks :)