I have never heard of not being able to seperate the flanges (metal part) from the hub (plastic/resin part that the tape actually loads onto). I have had so flange screws that were almost impossible to unscrew, or actually broke withn unscrewing, but once the screws are off the flange should easily lift off.
You mentioned 3 screws, some Scotch are 5 and 6 screw but that would be very obvious that you did not remove those screws. I cannot think of a reason why the flanges would be stuck to the hub unless someone thought it would be a good idea to glue them, or that someting from the tape has leached out over time and gotten between the flange and the hub that then turned into cement. Never heard of that eather.
If you have a long thin screwdriver I would try and see if you can get in wedged in between the flange and the hub and give a gentle twist.
Of course the razor blade trick will work, but it is still going to take you a bit of time to cut through 10" of tape, and some Scotch flanges had smaller windage holes which will not allow you to cut through all of the tape.
Commerical decks have a waste basket function which allowed you to wind the tape off the side into a waste basket, essentially bypassing the takeup tension arm shut-off sensor. You can do the same thing by lifting up the takeup tenstion arm and pressing play. Once you get the tape started into the waste basket it will continue to flow right in. Of course you are going to be standing there for quite awhile. If you have a flange that will come apart you would be better of winding the tape to that reel, taking in apart and then popping the tape off of the hub and repeating the process until you had all of the tape off the reels.
Travis