Adatre almost gets it right. Crank 3 must certainly be about the fact that while Chelios has been indestructible, but all of the excitement and abuse from the last 2 movies has caught up to him (as explained by Dwight Yoakam with a naked stripper sitting in his lap during the credits).
The Plot: While Chev was recovering Verona's 4th brother kidnapped Amy Smart while she was out getting medical supplies. Now Chev has to endure Crank 1&2 level batshit crazy chases, fights, and drug use all while keeping his heart rate or adrenaline level (notes from the studio will probably decide) below a certain level or his heart will explode.
The Title: Crank 3: Buzz Killed
This is a perfect How Did This Get Made movie, and the first one that I actually own and love.