first, after just 3 days a new tank would not be filtered, even if you moved gravel from a totally cycled tank it takes at the least a week before the bacteria is enough to break down waste matter produced by any fish. second, a six gallon tank is not a very big tank and fish, even though stores advertise that you can put mulitiple fish in a small tank, the fish usually will grow stunted or their life expectancy will be shortened.
if you do want to put multiple fish in, pick some kind of fish that doesnt get very big like neon tetras or guppies. but before you put fish in it is critical to cycle the tank completly. there are several ways to do this so read up onit and find the one that fits your requirments best.
good luck with the new tank!