If my memory serves me correctly, there should be 3 bolts in the pulley that hold it on a split collar. There should also be three empty holes as well. Unscrew those bolts and screw them into the other holes and this forces the pulley off. The collar is held on by a bolt thru the split.
One of the pulley's may appear in have been installed upside down, in other words, the 3 bolts are facing the deck. This makes that pulley line up with the center drive pulley. BTW, IH calls these pulleys 'sheaves'.
While you're fooling with one, check the condition of the others. Trouble comes in sets of 3.
Here's a link to the C-3 Manual: