Malachi 3:1 mentions two messengers: The first is John the Baptist, who prepared the way for the second Messenger, "the Messenger of the covenant," Jesus Christ.
Malachi doesn’t mention John the Baptist or Jesus. Those are just Christian reinterpretations.
The author of Malachi (literally ‘messenger’) was likely Ezra rather than someone actually named ‘Malachi’, and Ezra is probably also the ‘messenger’ intended at Malachi 3:1.