Livebearers are horrendously bad parents, even the females will drop a fry, then turn round straight away and eat it. That's why they have so many.
I personally wouldn't put your females in a breeding net, they will be stressed, and this could lead to problems with the fry, and possibly even the death of the female. Imagine if you were kept locked in your bathroom - enough room to move around but you'd hate it.
Much better for the hobbyist to let nature take its course. If you have a reasonable level of cover in your tank, then a few fry will survive, and you really don't want to be overrun with fry.
Remember, you have 3 females. They'll have, say, 20 fry each, so a total of 60. Then in 6 weeks, they'll have another 60. And then in another 6 weeks, another 60. By that time, perhaps half the original batch will have been female, so they'll be reaching reproductive maturity. So, instead of 60 fry every 6 weeks, you'll be getting 660. And after another 6 weeks, when the second batch of fry are mature, that'll be 1260 fry. And hopefully by now you can see that if you try to save every single fry, you will soon have a tank that resembles a tin of sardines. It's just not sustainable.
Let nature do what it wants, remember that these fish have been reproducing like this for millions of years. Don't project your mammalian nuturing instincts onto another species, that simply doesn't care if its young live or die.