The absolute minimum of females to males for any common livebearer is 2females per 1male, although the more females you have the less stressed they will be and thus the more balanced the pecking orders will be. The minimum tank size for 3mollys is 15gallons as they are very active fish with lots of personality and can grow to 4inchs long.
Mollys can produce huge amounts of fry, somtimes over 100, although their pregnancys are longer than guppys in general and can last up to 2months. Molly fry take up to a year to mature and the minimum advised fry tank size is 10gallons due to their large batchs of fry and how overstocked fry tanks can get in a single batch of fry produced by one female.
You will need to feed the fry at least 3 times a day(preferably with some sort of fry food like TetraMinfry or "first bites" by hikari" for the first couple of months) and the fry tank will need more maintanence than the adult tank as fry are very sensitive to water quality and enviromental conditions than their parents, to say the least getting the tank fully cycled is an absolute must.
Most lfs(local fish stores) will take mollys at around halfgrown size although this is by no means assured so if you want to breed them i strongly adsvise you look around your lfs's and see who takes fish off customers and under what conditions before you breed the fish, i wouldn't kid yourself you'll be making any money from breeding mollys as the vast majority of lfs's buy mollys, amoungst other livebearers, for pennys and have no insurance against sick fish when taken off customers/unoficial breeders so most won't pay anything for them but simply take them off your hands for free.
For a serious molly breeder though i'd recommend at least a tank of 25-30gallons with a group of 4-5mollys(1male per 3-4females) and a fry tank of 20gallons at least. Try not to buy heavily pregnant females or put them in breeding nets as this causes great stress for the pregnant fish and can end up with serious birthing complications like her aborting or even dying during giving birth.
Adding lots of densely planted areas in the tank will help increase fry survival rates and overall lower stress for the group and give the pregnant female somwhere quiet and hidden to give birth so she doesn't her chased by other fish who will want to mate with her and/or eat her fry.
For increasing the chances of a successfull birth and healthy fry, feeding the pregnant fish a variety of thawed out frozen foods like krill, daphinia, bloodworms and tubifex is good(you can also get them in freezedried form) and also feeding them blanched cucumber and/or algae wafers is a must as for a healthy molly they need a vegetable aspect of their diet, and keeping excellent water quality of course with a strong filter current. The more time you leave the tank lights off for the more time the mollys will get to relax or establish pecking orders as this is when they are their most active socially
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So just think it all out first. 
