Whats the different between male and female micky mouse platys?

they are yellow,red and,black (black at beginning of there tales) but one has no red that's on there top fin above there back but looks identical to the others in every other way. then the no red is that a difference between male and female? and i have 3 but one keeps picking on the other just one not both ganging up on one i don't know what to do he is not hurting him just chasing should i get a 4th platy?