When I was first learning to throw a change-up, I threw a three-finger change because my hands weren't big enough to grip a circle change-up. However, my biggest challenge with the three-finger change-up was my control.
Quite simply, I had a difficult time controlling the speed of the pitch, and as a result, often threw it far too hard for it to be effective most of the time.
Most of the college pitchers I coach have this same problem, too.
At that time, I was working with Coach Bill Thurston, a pitching consultant at the American Sports Medicine Institute (ASMI) in Birmingham, Ala. ASMI is a biomechanical research and development institute founded by Dr. James Andrews, one of the elite baseball surgeons in the country. ASMI also touts Dr. Tom House, Dr. Glenn Fleisig, and a long list of other baseball instructors and doctors who are tops in the field of baseball and baseball pitching on its research staff.
When throwing a change-up, Thurston taught me to shorten my lead-leg stride slightly (I'm talking a few inches here, not feet) and collapse on my back-side (again, slightly).
By collapsing on your back side, I'm talking about the back-side leg-action associated with your follow-through...
Instead of a high-up-and-over back-side leg-kick in the follow-through phase of the pitching delivery, which should be characteristic of a fastball release, a pitcher will still follow-through with the backside leg, but it will not be as "pronounced" as when a fastball is thrown.
Thurston said these pitching mechanical adjustments should be negligible to the naked eye.
He noticed Major League Baseball pitchers making these two slight adjustments only after viewing them on high-resolution video, which was slowed to 1000 frames a second.
Remember: the key to an effective change-up is deception. If a hitter notices different mechanical changes in a pitcher's delivery, the hitter will be able to recognize it and make the necessary adjustments.
The delivery, therefore, needs to remain the same as the fastball.
I recommend that you video-tape your pitching motion from the right-side and left-side during practice if you decide to implement these slight mechanical variations when throwing a change-up.
Also of note: when I switched my change-up pitching grip from a three-finger change-up to a circle change-up (in college when my hands became bigger), I did not need to vary my pitching mechanics at all when throwing the new off-speed pitch because the speed-difference (from my fastball velocity) resulted from the grip of the pitch – and from the slight, natural pronation of my throwing hand upon release.
Because of that, and the sharp down-and-in movement to right-handed hitters (I'm a righty), I found the circle-change to be a much better change-up pitch for me.
As a closer in the Chicago Cubs organization, the circle change became my strike out pitch. (120 K's in 80 innings as a pro pitcher. 68 of those K's were with the circle change!)