A 3 gallon is probably small for any danios-considering how active they are.
How about 5-6 celestial pearl danios? Yes, I know I just said probably no danios, but CPDs are technically not danios, nor even closely related. They largest females top out at just under an inch, with males smaller. They also do not need heat- just room temp. Just be sure to only have one male in a tank that small.
Or least killifish?
Or shrimp?