Hi.
Sorry to hear about your molly. You have to realize that any organism can suffer from a breakdown of one of its' body systems, in this case reproductive. She may have had an inflammation, an infection, she may have been old and some part of her not working as good as it used to, etc. An unfortunate but natural (at least usually) consequence. A couple of months ago I lost a Platy in a similar fashion. Since she was 2 years old (which I believe is on the old side for them) I just attributed it to her age.
While livebearers are nibblers, in that they need to feed more often than other fish, I'm wondering if you are over feeding them. (I'm assuming that you meant 3 times a day. You forgot to end your sentence
Feeding them once a day is more than enough. Live bearers will happily nibble on algae for the rest of the day. My tank, which contains guppies and Platies , usually gets fed only every 2-3 days, and they once went as long as 6 days without food. I have heard other people go for longer in between feedings, though of course, I'm ony comfortable with the limits I have set for myself.
Another thing to consider (IMO) is the diet. I do not belive in mono-diets. Like people, most fish need variety in their food. And it is easier to provide than you might think. Of course, other kinds of flake or pellet foods are available, as well as frozen or freeze dried foods. Insect larvae, certain worms and crustacea can be grown in a barrel outdoors (do not harvest wild ones). And the easiest thing you can do is add certain foods from your very own kitchen. (peas, lettuces, zucchini, and seafoods.)
The reason I bring all this up is that because a poor diet can contribute to disturbances of the digestive system. Some fish can become so constipated, it can actually kill them. Also, mono-diets may contribute to poor overall health, leading to infections and such.
Anyways, theres my 2 bits. I hope that helps somehow.
Chris