Depending on what kind of Mollies...Hardly, if any.
Sailfin Mollies are going to get too big for that tank, and they won't be that happy in there. I guess you could keep a trio of regulars, or maybe a trio of Balloons, but that's all I'd go for. I'd personally pick the Balloons, if you like them, seeing how they are smaller and therefore will be happier. I'd go for a 15 if you wanted to keep regulars, 25-30 for Sailfins.
They may be small now, but they will get bigger. I'm assuming you got Regulars, which can grow to 4". I'd have a plan for a bigger tank for them down the road.
Mollies do tend to eat some algae. They like greens. You can try blanching some small pieces of veggies for them, I'd sure they'd appreciate it.