The auto tranny has little 'pick and go' power at 65mph, so down shifting into 3rd will give you the extra power you need when intering a faster lane(the speed limit is 65 where I'm from, but people drive at 80mph-85mph). Do not keep it in D3 or else you'll see your fuel gauge drop fast.
The transmission was build to shift manualy, so doing this wouldn't hurt anything.