I'll start Fenech vs Nelson: 1991, 1992 and 2008 - 16 years gap B-Hop vs Jones: 1993 and 2010 - 17 years Larry Holmes vs Smith: 1984 and 199 - 15 years Larry Holmes vs Mike Weaver: 1979 and 2000 - 21 years. Hopefully you'll add another interesting examples
In February of '86 on the undercard of the first Azumah Nelson v Marcos Villasana title fight, several ex-champs appeared in professional 4 rounders to help a charity for Mexican earthquake victims. Ruben Olivares, Pipino Cuevas, Carlos Zarate and Rafael Herrera fought respective opponents. Herrera's opponent was Alfredo Menneses, who he had beaten in November of '66 in their second bout. So that is 19 years and 3 months between their second and third bouts.