Here is my opinion on shotgun ammo for 3 gun. I've only shot a few but around here 3 gun matches aren't constantly on clubs schedules like USPSA or IDPA.
If your shotgun accepts chokes any budget ammo that cycles your gun will be fine for a local match. A good TD won't make a stage where everyone has to use specialty ammo without at least putting out some kind of warning before hand. Without a choke you are better off using #6 shot or bigger (if they will allow it) but honestly budget ammo + modified choke is better than larger shot without a choke.
New gun, same ol' shot.