The 1inch ratio is not a rule, but a guideline, and has qualifiers on it, as 1inch of Dwarf Puffer or Pleco does not equal 1inch of Otocinculus.
Adding more to a 3 gallon tank is possible, but it takes a lot of work and research and dilligence. It's not advisable. There are pretty much NO fish you can add to the tank, but as the FAQ says, you may add an African Dwarf frog (as in, a single frog), or some Ghost Shrimp. However, the Ghost Shrimp needs very good living conditions, and bettas are prone to... you know, murdering them and such.