Since there's no such thing as "MERC III", that narrows it down.

Ford discontinued Mercon back around 97.

Everything uses Mercon V now that used to use Mercon, regardless of the recommendations made at the time the vehicle was made.

If your vehicle was made to use Mercon and that's all you can get, then it wont hurt anything to continue using a Mercon-rated fluid - you just won't get the benefits of the improved fluid.