There are lots of things to look at with your "trio". What kind of food the birthing females on eating, bedding, space/territory issues, other stressful things that may be going on around their enclosure, etc, etc.

Sometimes if only one female has a litter she can also become nervous or even so overprotective that she will kill them herself. We would need a lot more detail on your husbandry to even vaguely help you out.

Don't give up, sometimes nature is frustrating...other times its something simple you can change for the better.