You can put three mollies in a 10-gallon tank. As a general rule, an inch of fish requires one gallon of water. In captivity, mollies usually grow to be 3–4.5 inches long, so each fish will need around three gallons of water to swim freely.
However, if your tank is a community tank, then you also need to consider the other tank inhabitants and their spatial requirements.
You should also note that mollies are shoaling fish and seek safety in numbers. Therefore, you should keep at least three or four mollies in a tank. If you can get a bigger tank to accommodate more mollies, you should do so.