The best idea would be to ask to people who are already shooting rifles before you buy but bolt action will almost certainly be a waste of time for most 3-gun matches. You may see a match or two geared towards that but it is more of the exception than the rule. Semi-auto is the way to go. Most clubs have people who let other shoot their guns so you could probably get away with just bringing ammo (not reloads though) and testing the waters with someone elses gun.
Both 223 and 7.62x39mm will make minor PF. 223 is the universally better choice because it shoots flatter, has less recoil and and better quality guns are chambered for it. 7.62x39mm is fine though if you already own one or are shooting for fun.
New gun, same ol' shot.