If you do decide to keep just 3 goldfish in the tank they will tend to grow nice and big. The more water goldfish have the more they grow.
I have a female betta in with my goldfish right now and they seem to get along just fine. Might not work in all cases, especially if the goldfish have become defensive of their territory by now. The longer they've been in there, the more likely that will be the case. Male bettas can sometimes live with them, but there have been cases where the bettas killed the goldfish. Siamese Fighting Fish is the betta's other name, after all. The females are hardy and peaceful and require very little care. They come in many beautiful colors and would be a lovely addition to any tank.