In modern boxing, when the champion in a division wins the title in a higher weight classification, he must give up the crown he has won at the lower weight. For example, if the bantamweight champion (118-pound limit) defeats the featherweight champion (126-pound limit), he must give up his bantamweight title.
Before this rule was instituted, a boxer named Henry Armstrong was permitted to hold the three championships he had won. From August to December 1938 Armstrong was the recognized champion of the feather weight, lightweight and welterweight divisions.
From The Giant Book of More Strange But True Sports Stories by Howard Liss. Illustrations by Joe Mathieu.
https://en.wikipedia.org/wiki/Henry_Armstrong
Liss got the timeline wrong. Armstrong won the featherweight title on 10/29/37, the welterweight title on 5/31/38 and the lightweight title on 8/17/38.