I hunt geese with my 3"Mag.
BBB is what I use of Greater Canada geese.
BB for lesser Canada geese.
For Snow geese, you can go with #2 shot.
All shot must be steel, for waterfowl in the United States and Canada.
I always use a full choke when hunting any waterfowl, but I am pretty fair shot.
It sounds like you are new to this, so I would go with a modified choke.
The full choke gives a VERY tight pattern that a regular shooter should stay clear of if you want to fill you bag.
Good luck and send me an e-mail if you want more info.
Waterfowl hunting has been my passion for 45yrs.
Eric