303 is the oldest model. 390 replaced it and IMO is a much better design. 391 is an upgrade to the 390, some people who have the 391 love them and some would rather have the 390. 391 has a slimmer forearm than the 390. I have a 390 and 391 Xtreama which is different than the standard 391 so I can't be specific about the small differences between the 390 and 391.
More ammo! I need to shoot something!