- Messier object 3, a globular cluster in constellation Canes Venatici.
- The M3 Lee was a U.S. Army tank during World War II. When built in Britain, it was named M3 Grant.
- M3 SMG is also the model number of an American submachinegun
- M3 can also mean the highway in England, see M3 motorway
- M3 can also mean the proposed motorway in Ireland, see M3 motorway (Ireland)
- M3 in economics refers to the amount of cash in circulation, the amount in checking or demand deposit accounts plus savings accounts, money market accounts, CDs and foreign-currency holdings. See money supply.
- The M3 Bradley is a cavalry fighting vehicle for the U.S. Army.
- BMW M3 is an automobile model manufactured by BMW.
- M3 and m3 refer to major third and minor third in music, respectively.
- M3 was the code name for a special American military mission on Roosevelt Roads Naval Station, in Ceiba, Puerto Rico
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