With the 3-speed, push in the clutch, shift to higher gear, let out the clutch. Downshifting is the same procedure. Except for 1st gear, can't downshift from 2nd to 1st. 1st doesn't have a synchronizer. The 3-speed doesn't have a lever on the gear shift to go into reverse like the 4-speed. Some floorboards also have a different hole configuration between 3-speed and 4-speed transmissions.
Rick
