First off, search this forum for similar posts...you'll find a ton of good advice. Main point-check your pattern. You'd be surprised at the holes that result from some gun, shell and choke combinations. What that here's some suggestions...

Lead shot:
-Winchester Supreme 3" no. 4, 5 or 6 shot
-Federal Premium Mag-Shok 3" no. 4 or 6

Non-toxic($$$)
-Hevi-shot 3" no. 4 (a strong fave)
-Remington Wingmaster HD 3" no. 6 (devastating)

There are other turkey ammo mfgrs out there; Kent & Nitro are two that come to mind, but I've never shot them.

If you want to go with a 3.5" shell and your gun will take them, go ahead. But...a 3" shell will do the job with enough authority, provided the bird's in range.

When I first got the fever, the only turkey loads I could find were 2.75" Win Supremes...they worked. I've had great sucess with Hevi-shot and Rem W.M. HD