We had a similar problem when we first got our platys. The smallest one (Minnie) had to settle for what Daisy & Belle didn't get to.. unfortunately, she had to learn to go after the food more aggressively but she did learn and now gets as much as the others. One thing that might help is to feed your swordtails like we do the platys.. we take their pinch of food between our fingers, put them just below the surface and rub the food between our fingers as we move our hand across the tank. This way the flake pieces or pellets get distributed across the tank and everyone gets a better chance for a fair share.. maybe start where the other 2 are and move towards Flo.
I hope it'll work, best of luck with them.
