I seem to recall there was a transmission firmware update for the early 451s which was to partially address this issue. I would hope that after this many visits to the dealer that it has been applied.
I think there was also an issue with some early clutch disks, but after 3 clutches I can't imagine it would still be your problem.
Have the pilot & release bearings been replaced each time? Has the clutch hub spline been lightly greased at replacement? If the transmission input shaft has any rust or damage to the splines it can prevent the clutch disk from sliding freely. Unlikely you would know if these were done properly by the shop. Possibly your repair order may list the pilot & release bearings as items.
Has the transmission fluid level been checked or replaced?
~toaster
I think there was also an issue with some early clutch disks, but after 3 clutches I can't imagine it would still be your problem.
Have the pilot & release bearings been replaced each time? Has the clutch hub spline been lightly greased at replacement? If the transmission input shaft has any rust or damage to the splines it can prevent the clutch disk from sliding freely. Unlikely you would know if these were done properly by the shop. Possibly your repair order may list the pilot & release bearings as items.
Has the transmission fluid level been checked or replaced?
~toaster