Both Muhammad and Joe were equally motivated for all three fights. And they would have been had they fought TEN times.
Frazier was certainly motivated to beat Ali up, but he was older, but then again so was Ali. The real story is that Ali paced himself better, didn't go for the KO and used holding and pulled Frazier's head down to negate Frazier's inside work. It's not really an exciting story is it?