Battlefield 3 will feature 4 classes (or kits), much like previous Battlefield games. Developer DICE feels this is the sweet spot in terms of accessibility and gameplay. The classes/kits in Battlefiled 3 can therefore be expected to be virtually the same as they were in Bad Company 2. The classes in Battlefield 3 were fully revealed during E3 2011.

ASSAULT

The Assault class carries a primary weapon consisting of an assault rifle — like the M4 and AK-47 — and is now merged with the medic class — the Assault class can now heal and revive teammates. It can also function as a more offensive class with ability to swap out the revive kit for a grenade launcher.
The confirmed weapons for the Assault class so far are: M16A4 assault rifle for USMC, AEK-971 for Russian side.
M9 sidearm for USMC, MP-443 for Russian.
Grenade launcher or defibrillator kit.

SUPPORT

The Support class returns to its role it had in Battlefield 2, with its main purpose to provide heavy fire from a Light Machine Gun (like an M249 SAW), and resupply teammates with ammunition. In Battlefield 3, the Support class is broadened with the ability top use lighter weapons and assault rifles like the M4, with several attachments, including a powerful flashlight which has the ability to blind enemies in dark places.

The confirmed weapons of the Assault class so far are: M249 SAW for USMC, PKM or RPK for Russian side.
M9 sidearm for USMC, MP-443 for Russian.
The ability to use lighter weapons with attachments like the M4 or AK-74U.
Ammo pack to resupply teammates.
Ability to mount LMGs on surfaces for greater accuracy.

ENGINEER

The Engineer class in Battlefield 3 is very similar to the class in Bad Company 2: its primary role is to engage tanks and armored vehicles with an array of rocket launchers such as the RPG-7 and the AT4, or use anti tank mines. The primary weapon is a carbine rifle, with a variety of unlocks available, ranging from shotguns to sub machine guns.

Main heavy weapon includes the AT4 for USMC and RPG-7 for Russian side
Primary weapon is a carbine (like an M4 or AK-74U).
Rocket launchers can be swapped with anti tank mines.
Repair tool for repairing friendly vehicles.
M9 sidearm for USMC, MP-443 for Russian.

RECON

The Recon class carries a sniper rifle for long range engagement, but for Battlefield 3, the Recon class has a more active role as well. The Recon also has the ability to call in artillery strikes (expected unlock). Furthermore, the Recon can equip C4 explosives to blow up enemy vehicles or map objectives.

Recon weapons: Semi automatic sniper rifle, MK11 for USMC, SVD for Russian side.
Will have a wide variety of team-play oriented gadgets.
C4 explosive pack.
M9 sidearm for USMC, MP-443 for Russian.