Assuming your tank is filled to within 1" of the top and that you have sufficient substrata (gravel) at the bottom in which to grow plants, say it's 2" deep, then your tank is missing about 3" of its water holding capacity If your tank is 12" deep you have lost close to one third of its water ie almost 1 gallon. That means your actual quantity of water is just over 2 gallons. The basic rule for fish keeping is 1" of fish needs 1 gallon of water. A Male Betta grows to about 2" in body length. This means that your tank is holding its maximum fish capacity already with 1 male Betta in it. The answer to your question therefore is. You can 'safely' put no more fish into your 3 gallon tank with your Betta.
you can put a betta in a 5 gallon tank
Only 1. Usually a Betta is all that is kept in a 3 gallon tank
A betta fish would be much more suited to a 1 gallon tank than a 'variety'. You shouldn't keep much more than one fish per gallon anyway.
A 1 gallon tank could have one betta fish, but you will need to be diligent about changing the water on a regular basis. The smallest tank that should be used, even for a betta, is a 2.5 gallon tank as it at least gives the fish swimming room.
None are recommended if the Betta is a male.
Go fish
Any.
No
Yes.
yes of course! my betta is in a 2.5g and is perfectly fine
The fish are not bad together. But way to many fish for a ten gallon tank. You put one fish for every gallon. They will probly do fine but you will have to clean more.
Just one male betta, but you should consider a proper betta set up which can have 2 to 2.5 gallons for the one fish.