You should bring enough ammo to do the course twice over if neccessary (reshoots, range equipment failures, fun blasting after the match). Of course if you don't know the size of the match it can be tough to judge. I always err on the side of safety, and so you can help other shooters out too if they run low. Alot depends on the size of the stages and the targets used.
As a rough rule of thumb I might say 75-100 rounds per stage for rifle and pistol.
For shotgun birdshot is generally okay for steel targets as long as the range is not too long and you shoot well. Buckshot gives a bit more margin for error in knocking things over, but also more recoil. I usually split the difference with "tactical" buckshot in 00 or #4. But at the match this weekend I had to borrow a round of birdshot to give myself the best chance at an aerial target that started the stage. Say 40-50 shotgun rounds per stage.
Slugs could be neccessary, but the match info should really point that out ahead of time. Maybe take 10-20 slugs just to be sure.
Hope this helps, after your first match you will know the drill. The good thing about 3-gun is that the rifle and shotguns all use about the same ammo so one can usually find some extra ammo if needed.