definitely not 3 females in a 10 gallon, too small a space, and fighting IMO would happen. i personally would never house a male and female betta splendens together, regardless of the tank size, unless i was breeding them.
considering the OP's grandma wanted to mix male and female bettas in one tank, i suggested common and easy to care for tetras that can usually be found locally (the neons for sure) and are not that expensive. once grandma has gained some more knowledge on proper stocking, and wants to take her tank in new directions, then i highly suggest branching out to the less common and more unique fish that would fit for her tank size.
and just to clarify, i would only do the tetras in this tank if the otos were not increased in number at all. going back to that 'small tank, small stock level' guideline i follow.