One betta only in a 3g.
My advice would be to keep your 3g as an isolation tank if you have a sick fish that you have to separate, or to quarantine a new fish. It is really too small to be a permanent home for any fish. Bettas do love space, and when they have it they are not an aggressive fish. I have had three female bettas quarantined in an 8g. They went fine for 3 weeks, then one of the females decided that tank was her territory. I cut the quarantine time short and put all three in the 50g community tank, no more aggression.
I currently have a betta in a 12g with 10 cardinals. They get along great.