RE: What 3 inch 6 shot shell?
You're going to have to shoot it bud, no way to tell...
Not only do you have to look at pellet delivery, some shells don't center the pattern like others do...
I'd start with Winchester Supremes and Federal Premiums, in a standard full choke I'd stick with #6s for more pellet count and inside of 40 yards #6s will kill them dead...
You might be suprised how well that gun groups, I've got a double barrel 12gauge that I used 30 years ago, with Remington high brass 6s, I killed several out to 35 yards...
It doesn't take 15 pellets in the neck/head area to kill a turkey...4-5 will do just nicely...