3 quarts overfull? That is insane. I wouldn't trust one more word that comes out of this guy's mouth.
Does he really think that Ford got it that wrong?
I'd like to talk to this idiot.
If I had to guess, I'd say that he lost/damaged your original dipstick or tube. He found one that was close, but not exactly. He realized that the trans is low enough to start slipping, when the dipstick shows full. So, he told you to keep it above the full line. There is a part number stamped on the dipstick, so that should be easy to check. The tube, I'm not sure about. If it did have a part #, it would probably be on the little bracket that holds it in place, and may be impossible to see (while in the car).
If you are just low enough that the trans slips every now and then, when the dipstick shows full. Then 3 qts more may be too much. My old GF's Fiero wouldn't slip until it was 2 - 2 1/2 qts low (it leaked, and she let it get low enough to slip quite often). But, I used to have a C4 trans (Ford 3spd RWD) that leaked a lot also. From where it started slipping, to the full level, was just over 1 qt.
I think your owner's manual tells you to add fluid in 1/2 pint increments (1/4 quart), until it's in the full range. I have always been under the impression that overfilling a trans is just as bad, if not worse, than underfilling.