Three finger changeup
To throw an effective three-finger changeup, center your ring, middle, and index fingers on top of the baseball (as shown in the third picture at right). Your thumb and pinky finger should be placed on the smooth leather directly underneath the baseball (as shown in the middle picture).
A lot of pitchers that I work with like to "touch" their pinky and thumb when gripping this pitch (as shown in the middle picture). It helps to develop a good "feel" for the pitch, which is important since the changeup is a finesse pitch.
OK, now that you've got your grip, hold the baseball deep in the palm of your hand to maximize friction and to "de-centralize" the force of the baseball when the pitch is released. This helps take speed off of the pitch. Throw it like you would a fastball: Same mechanics. Same arm speed. Same everything.
One way to develop "fastball mechanics" but changeup speed is to practice throwing your changeup as you long toss (throwing beyond 90 feet). Alternate fastballs and changeups at 90-or-more feet for about 20 throws a couple of times a week.
Note: Advanced pitchers can experiment with "turning the ball over" to create even more movement on the pitch. To do this, pronate your throwing hand (turn it over as if you were giving your catcher a "thumbs down" hand signal) as you throw the pitch.
Resource from Thecompletepitcher.com/pitching_grips.htm#q3