Could 3 female bettas live in a 3 gallon tank or is that way too small?

Yes, she can. A tank that size could even accomidate her and an otocinclus (i'm not sure of the spelling) which is a type of algae eater. As long as the water is conditioned, water changes are done on schedule and there is adequate lighting, and there is air going into the water with or without the filtration things should be fine. You will find the algae eater a small, friendly and welcome addition. A tank this size with lighting a good part of the day will quickly grow algae. Its healthier to control it naturally than to try to control it with chemicals. Best of luck!