You will be just fine with a 3" gun. if you are on a limited account, as many beginning hunters are, buy yourself some cheap 3" BB's and have fun shooting geese. These are more than enough for geese inside 40yds. Just remember to limit your shots and you will take geese.
If you can spend a bit more, buy some higher $$$ shells and get some 3' Hevi-Metal in 2's or BB's and you just extended your range 10 yds but at almost 2x the cost. Either way, hunt smart, not hard and you will take many geese and have fun while doing it and not be broke and beat up.