hoping someone can tell me about the 3 gun match at ben avery.   having a hard time finding any good info on it

It is on the 4th Sunday of each month, which would be this weekend. Starts at 9:00 AM. You can shoot just pistol, or just rifle, or just shotgun (although I've never seen anyone use only a long gun), or any combination. Scoring is Cactus League practice rules, time minus bonus seconds for A hits, plus time penalties for misses or no shoots. 4 pistol stages, 2 rifle stages, 2 shotgun stages. 100 rds of pistol ammo is usually enough, but sometimes the long gun stages start with pistol so you would need more. 100 rds of rifle would be plenty, and I don't think you'd need more than 2 boxes of shot shells.

You can see a schedule and match results at CCML.

what kind of shotgun shells would be good

Birdshot for the poppers. You might need a few slugs too.

FWIW, there are a couple of good 2-gun (rifle/shotgun) matches in town (in case you don't already know). South Mountain R&P club has theirs this Sunday. Sign-up at 7:30am, shooting starts at 8am.

Rio Salado Sportsmans Club has theirs on the second Saturday of each month.

Both are good practice and a fun time if you want to concentrate on long guns.