Everyone knows about Henry Armstrong....not only the boxing champ at 3 weights, nor just because it was simultaneous...but there were only 8 weight classes at the time (and only 1 title organisation)!

He held the featherweight (126), lightweight (135), welterweight (147) belts simultaneously....I learned this week he later unsuccessfully tried to go up to middleweight (160).  From Wiki:  "he tried to make history once again by becoming the first boxer to win world titles in four different categories in a rematch with Garcia, already the World Middleweight Champion, but the fight ended in a ten round draw, Armstrong's attempt to win a fourth division's world title being frustrated. According to boxing historian Bert Sugar, many felt Armstrong deserved the decision in this fight."

34 pound gap!  Must have had an iron chin and will because power has physical limits.