Its just 3rd gear... when would you use it? - If your going downhill and don't want the trans to shift to overdrive. Or if you want to pass at 50+mph, you could manuelly shift it there so you won't have to wait the split second for the Auto to do it itself when you get out to pass.
So basically, leave it in 'D' most of the time unless you want to accelerate quickly at highway speeds or you want the increased engine braking of leaving it in a lower gear. Putting it in '3' prevents the transmission from shifting into 4th.
"Yeah, I hear ya! I've got an auto 3.5 Alty SE and I only drop it into 3rd when I'm at a complete stop. Sounds nerdy, but I'm not going to risk cooking my new transmission until I know for sure doing it while the car is rolling won't hurt the tranny. However, 3rd really gets those revs up and running, so those other cars best move out of my way.... ;-)" So wutz he mean by that???