I wouldn't put ANY platies in a 3gal. Platies are fairly active fish that breed constantly, so they need space, not only for themselves, but for any potential babies. I have two female platies and a female swordtail in my 55gal right now and will be adding a male swordtail as soon as I can find one I like (just lost my beautiful pineapple swordtail male). I honestly wouldn't have platies in anything less than a 10gal and even that is pushing it.
Like tom said, research the fish you are interested in before you buy them. NEVER rely on your LFS, LPS or Petsmart/Petco to give you a straight answer on stocking because they are in the business of selling you fish, regardless of what is best for the fish. Also, if they have you overstock your tank on their recommendation and the fish die, you come back and buy more fish, so they make even more money. Whether it's coming onto a forum like this one or just googling on the internet, RESEARCH is your best friend. I saw that my LFS had horseface loaches and fell in love with them...instead of buying one right then and there like I wanted to, I went home and did a lot of research on them because I knew nothing about them. Ended up I could definitely fit one into the stocking plan for my 55gal, but if I had a smaller tank, I probably wouldn't have been able to get one (they get to be 8" long). I wonder how many people bought one of those loaches thinking "Awww! Look how cute and little they are (was about 2.5" when I bought him)!" and will be horribly surprised when that cute "little" loach gets to be almost as big as their tank.