Don't let the dealer tell you that the problem is normal. It's not. If you need to go as far as writing a letter to Ford, go ahead and do that. Downshifting from 3rd to 2nd shouldn't be a problem unless you are spinning to many rpm in 3rd. I've never had this problem on my car. The only one that should be difficult is the 2nd to 1st shift. Once in a rare while I'll have the 2nd to 3rd brick wall and I used to have a problem with the 3rd to 5th shift (it would grind and clunk), but I just stopped doing that.
You should be able to spin up around 3500rpm in 3rd gear and shift into 2nd gear no problem.
2006 Ford Mustang V6 Pony Package (Sally)
2009 Mercury Milan
1992 Mustang GT (auto)
1986 Ford Bronco XLT (Papa)