If you don't want to breed, why would you want 2 females to 1 male? Males can live happily together as can females, and they live pretty happily all together. Color does not make a difference except to your eyes. They are the same fish on the insides, whether that be male or female. And for your final question, you have to intervene to have babies, so if you don't want babies just let the parents eat them as they would in nature. However, if you change your mind and you do want babies, you will need a breeding box/net and house the very pregnant female in it for 24 hours or less. You will need to see her gravid spot (a dark spot on her abdomen which may be difficult to locate with brilliant colored platies) before you place her in a box or net. Once she's had all her babies (she'll look very slender) you will need to remove her so she doesn't eat them.
Does this answer all your questions?
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Phishie
"An ounce of prevention is worth a pound of cure."