SureFire's term for a flashlight 'head' is "Bezel".
The bezel used by the M3 can be purchased individually by ordering a "Z46" (or Z46-CB from the Crenelated Bezel aka "Strike Bezel" version) from SureFire or any SureFire Dealer.
The Z46 is used by so-called Millennium Series models including:
M3 Millennium CombatLight
M500A Millennium Forend WeaponLights
M900A Millennium Vertical Foregrip WeaponLights
M96 Series Millennium Universal WeaponLights
M100 Series Millennium Handgun WeaponLights (discontinued)
Some later "9V" Shotgun Forend WeaponLights (those making use of the LM90 Lamp Module)
Other bezels that fit the so-called "Millennium threads" include the KL2 and KL6 (both discontinued) LED bezels, and the Millennium TurboHead.
Note that the Millennium TurboHead is not called "KT4" (although some people mistakenly do). KT4 is "Kit, TurboHead, No.4" featuring the Millennium TurboHead and the MN15/MN16 Lamps for three-SF123A models. The KT4 TurboHead Kit is used to convert:
M3 to M3T
M500A to M500AB
M900A to M500AB
M96 to M97