I will say that if you are going to keep the betta alone and it is you feeling the need for tank mates for him, not him, then a 3 gallon tank is okay. But not if you are going to put in even one more tiny fish, then it needs to be at least a 5 gallon.
In a 5 gallon he can have 2 otos but it will need real plants and to be cycled FIRST.
In a 10 gallon he can have 3 small cories or 4 otos.
Other fish that can be put with them in different sized tanks are small plecos, kuhli loaches, dwarf loaches, cardinal tetras (not neons), ONE Mystery or Apple snail.
Fish and Inverts to avoid: Barbs, Gouramis, danios, neons, guppies, mollies, platies, shrimp of all types, sharks, swords, rasboras, hatchets, angels, crab, lobster, any other fish with long or flowing fins, any other semi-aggressive or territorial fish,
goldfish.
some of the fish listed are hostile to the betta and some are in danger from the betta. Some are questionable and best not to try unless you are willing to lose one of both of them.