If you've never shot 3-gun...why not whatever 12ga you have? I don't think I'd drop $3-4k into a sport I don't know I'd like. Shoot, a NEF pardner can hold 7 rounds bone stock (5 in tube, 1 in chamber, and 1 ghost loaded) for $200. Add a gg&g improved follower for $30 and you've got plenty of money left over for some proper shell holders.
If you're just looking for an excuse to buy a tactical semi-auto 12 and have cash to burn; check out what they use at comps. Personally, IF I were buying a gun for a something I never tried and didn't want to use a pump, I'd use a cz712 with a nordic tube for less than $400....but thats me.
Oh yeah, before someone misreads that post: I meant $3-4k in 3 new guns and gear...not $3-4k in just that shotgun.