No one else in England can still do the same
The Reds belong in the elite club of just 7 teams to have ever won three major trophies in one season. The treble winners of 1998-99 made it possible. United are also the only English side to retain the bragging rights:
It's notable that Jose Mourinho is a treble winner himself. He guided Inter Milan to that achievement in his second and last season at the Giuseppe Meazza Stadium.