I had ten Cardinal Tetras in a 15 litre / 3 UK gallon tank a while back before I knew any better. They must have been a bit cramped but I managed to keep them alive. They are now in a 20 gallon tank which is much better for them.
I agree with Aernympha that you might get away with 6 Neons but nothing else. You would need to make sure you have good filtration and keep a close eye on the water parameters.
You could put six in there but they would all probably die. Neons are very delicate fish (meaning that they mysteriously die for no apparent reason). I would get something less delicate as a 3 gallon tank can have large shifts in temperature and water parameters very quickly.
platies are livebearers and very hardy. they come in a couple of colours. they're jolly little chappies . they're kinda rounded and have short tails. have a look at this http/fish.orbust.net/platies.html