I agree. A 3 gallon really isn't suitable for any fish in my opinion.
And besides lack of oxygen all the toxic chemicals will build up to higher levels way faster in a smaller amount of water. Your fish's lifespan will be dramatically shortened due to the constant stress of high levels, cramped conditions, and the insecurity of not being in the proper school. You will also never see their true behavior. I say just keep the frogs and snails.