So assume the patriots win the super bowl and the celtics win the nba title. When was the last time one city had three reigning champions? has it happened?

copied from some website

no city has ever held three such sports titles simultaneously.

the closest a city has come to having the bragging rights to three champions at one time fell 3 points short in New York in 1933, in the days before there were pro basketball champions. That's the year the hockey Rangers won the Stanley Cup from the Toronto Maple Leafs in the spring, the baseball Giants won the World Series from the Washington Senators in the fall and the football Giants just missed winning the National Football League title in the winter, dropping a last-minute 23-21 decision to the Bears in Chicago.

ps the article was from 1988 ...so it may be wrong if it happened since