Yeah, I agree with most of what the person above me said.. I've been breeding livebearers for years and years, and here's what I'd say:
* they can have anywhere between 8 and 200 fry.
* the amount of fry usually increases with each drop (i.e. a female's first drop ever may be only 18 fry.. but her 3rd or 4th may be close to 100, if not more)
* they began to breed at around 3 months if you give them the chance, but they're fully mature at about 4 - 6 months.
* as soon as the female drops, the male will try to knock her up again (and even without him she may have more. One knock up from a male can lead to 1 - 3 drops).
* if it's just those 3 swordtails in the tank, and you have it well planted, with some gravel, and you keep the fish well fed.. you may get away with several fry reaching maturity. It's best to seperate the fry if you can though, at least until they are a month or two old.
I would ask if you have a regular trio (1 male 2 females) or reverse trio (2 males 1 female). If you have the later, you should consider doing something about it, as that mix can stress the female literally to death. "Overworked" will take on a new meaning for her, if you know what I mean.