Battlefield 3 Cheats for PC

To hold grenades in position before throwing them, press the throw grenade button, then straight away press and hold the fire button. It will then be held in place until you release fire and throw it.

Below is a guide for how much XP/Score you need to get each of the upgrades for Battlefield 3's vehicles:
MBT:

IR Smoke: 800 MBT Score
Coaxial LMG: 2,100 MBT Score
Autoloader: 4,400 MBT Score
Zoom Optics: 7,600 MBT Score
Maintenance: 12,000 MBT Score
Coaxial HMG: 17,700 MBT Score
Proximity Scan: 24,700 MBT Score
Guided Shell: 33,200 MBT Score
Thermal Optics: 43,200 MBT Score
Thermal Camo: 54,800 MBT Score
Canister Shell: 68,100 MBT Score
Reactive Armor: 83,200 MBT Score
CITV Station: 100,000 MBT Score

IFV:

IR Smoke: 800 IFV Score
ATGM Launcher: 2,300 IFV Score
Belt Speed: 4,700 IFV Score
Coaxial LMG: 8,300 IFV Score
Thermal Optics: 13,100 IFV Score
Proximity Scan: 19,200 IFV Score
Zoom Optics: 26,900 IFV Score
Maintenance: 36,000 IFV Score
APFSDS-T Shell: 47,000 IFV Score
Thermal Camo: 60,000 IFV Score
Guided Missile: 74,000 IFV Score
Reactive Armor: 90,000 IFV Score

Mobile AA:

IR Smoke: 400 Mobile AA Score
Anti-Air Missile: 1,200 Mobile AA Score
Belt Speed: 2,500 Mobile AA Score
Zoom Optics: 4,400 Mobile AA Score
Proximity Scan: 7,000 Mobile AA Score
Thermal Optics: 10,000 Mobile AA Score
Air Radar: 14,000 Mobile AA Score
Maintenance: 19,000 Mobile AA Score
Thermal Camo: 25,000 Mobile AA Score
Reactive Armor: 32,000 Mobile AA Score

Attack Helicopters:
IR Flares: 300 Attack Helicopter Score
Heat Seekers: 800 Attack Helicopter Score
Stealth: 1,600 Attack Helicopter Score
Autoloader: 2,800 Attack Helicopter Score
Zoom Optics: 4,400 Attack Helicopter Score
Proximity Scan: 6,500 Attack Helicopter Score
Air Radar: 9,000 Attack Helicopter Score
Guided Missile: 12,000 Attack Helicopter Score
Extinguisher: 16,000 Attack Helicopter Score
Maintenance: 20,000 Attack Helicopter Score
Thermal Optics: 25,000 Attack Helicopter Score
Laser Painter: 30,500 Attack Helicopter Score
Below Radar: 37,000 Attack Helicopter Score
ECM Jammer: 44,000 Attack Helicopter Score
Guided Rocket: 51,000 Attack Helicopter Score
TV Missile: 60,000 Attack Helicopter Score

Scout Helicopters:
IR Flares: 400 Scout Helicopter Score
Heat Seekers: 1,200 Scout Helicopter Score
Stealth: 2,500 Scout Helicopter Score
Belt Speed: 4,400 Scout Helicopter Score
Proximity Scan: 7,000 Scout Helicopter Score
Air Radar: 10,000 Scout Helicopter Score
Extinguisher: 14,000 Scout Helicopter Score
Maintenance: 19,000 Scout Helicopter Score
Guided Missile: 25,000 Scout Helicopter Score
Below Radar: 32,000 Scout Helicopter Score
Laser Painter: 40,000 Scout Helicopter Score
ECM Jammer: 48,000 Scout Helicopter Score

Jets:

IR Flares: 300 Jets Score
Heat Seekers: 700 Jets Score
Stealth: 1,500 Jets Score
Belt Speed: 2,700 Jets Score
Proximity Scan: 4,200 Jets Score
Rocket Pods: 6,200 Jets Score
Air Radar: 8,600 Jets Score
Extinguisher: 11,600 Jets Score
Below Radar: 15,000 Jets Score
Maintenance: 19,000 Jets Score
Guided Missile: 24,000 Jets Score
Beam Scanning: 29,000 Jets Score
..

To unlock more items in each class's kit, you need to play as that class to gain XP in the particular area. Below is a guide to how much XP/Score you need to unlock each upgrade:
Assault:
Defibrillator: 7,000 Assault Score
M416: 13,000 (20,000) Assault Score
M320: 21,000 (41,000) Assault Score
AEK-971: 31,000 (72,000) Assault Score
F2000: 41,000 (113,000) Assault Score
M26 MASS: 53,000 (166,000) Assault Score
AN-94: 66,000 (232,000) Assault Score
G3A3: 441,000 and 232,000 Assault Score
KH2002: 126,000, 441,000 and 232,000 Assault Score
Engineer:
FIM-92 Stinger: 7,000 Engineer Score
SA-18 IGLA: 7,000 Engineer Score
M15 AT Mine: 13,000 (20,000) Engineer Score
SCAR-H: 21,000 (41,000) Engineer Score
EOD Bot: 31,000 (72,000) Engineer Score
A-91: 41,000 (113,000) Engineer Score
G36C: 119,000 (232,000) Engineer Score
SG553: 278,000 and 232,000 Engineer Score
Support:
M249: 6,000 Support Score
C4: 11,000 (17,000) Support Score
PKP Pecheneg: 18,000 (35,000) Support Score
M224 Mortar: 25,000 (60,000) Support Score
M240B: 40,000 (100,000) Support Score
M18 Claymore: 40,000 (140,000) Support Score
M60E4: 60,000 (200,000) Support Score
Recon:
SV98: 6,000 Recon Score
T-UGS: 12,000 (18,000) Recon Score
SKS: 18,000 (36,000) Recon Score
SOFLAM: 27,000 (69,000) Recon Score
M40A5: 36,000 (99,000) Recon Score
MAV: 47,000 (146,000) Recon Score
M98B: 57,000 (203,000) Recon Score
M39 EMR: 252,000 and 203,000 Recon Score

For a complete list of all the Dog Tags that can be collected in EA's Battlefield 3, see below:
Default Dog Tag - Blank Dog Tag Available To All
Private First Class Dog Tag - Reach Rank Private First Class
Private First Class 1 Star Dog Tag - Reach Rank Private First Class 1 Star
Private First Class 2 Star Dog Tag - Reach Rank Private First Class 2 Star
Private First Class 3 Star Dog Tag - Reach Rank Private First Class 3 Star
Lance Corporal Dog Tag - Reach Rank Lance Corporal
Lance Corporal 1 Star Dog Tag - Reach Rank Lance Corporal 1 Star
Lance Corporal 2 Star Dog Tag - Reach Rank Lance Corporal 2 Star
Lance Corporal 3 Star Dog Tag - Reach Rank Lance Corporal 3 Star
Corporal Dog Tag - Reach Rank Corporal
Corporal 1 Star Dog Tag - Reach Rank Corporal 1 Star
Corporal 2 Star Dog Tag - Reach Rank Corporal 2 Star
Corporal 3 Star Dog Tag - Reach Rank Corporal 3 Star
Sergeant Dog Tag - Reach Rank Sergeant
Sergeant 1 Star Dog Tag - Reach Rank Sergeant 1 Star
Sergeant 2 Star Dog Tag - Reach Rank Sergeant 2 Star
Sergeant 3 Star Dog Tag - Reach Rank Sergeant 3 Star
Staff Sergeant Dog Tag - Reach Rank Staff Sergeant
Staff Sergeant 1 Star Dog Tag - Reach Rank Staff Sergeant 1 Star
Staff Sergeant 2 Star Dog Tag - Reach Rank Staff Sergeant 2 Star
Gunnery Sergeant Dog Tag - Reach Rank Gunnery Sergeant
Gunnery Sergeant 1 Star Dog Tag - Reach Rank Gunnery Sergeant 1 Star
Gunnery Sergeant 2 Star Dog Tag - Reach Rank Gunnery Sergeant 2 Star
Master Sergeant Dog Tag - Reach Rank Master Sergeant
Master Sergeant 1 Star Dog Tag - Reach Rank Master Sergeant 1 Star
Master Sergeant 2 Star Dog Tag - Reach Rank Master Sergeant 2 Star
First Sergeant Dog Tag - Reach Rank First Sergeant
First Sergeant 1 Star Dog Tag - Reach Rank First Sergeant 1 Star
First Sergeant 2 Star Dog Tag - Reach Rank First Sergeant 2 Star
Master Gunnery Sergeant Dog Tag - Reach Rank Master Gunnery Sergeant
Master Gunnery Sergeant 1 Star Dog Tag - Reach Rank Master Gunnery Sergeant 1 Star
Master Gunnery Sergeant 2 Star Dog Tag - Reach Rank Master Gunnery Sergeant 2 Star
Sergeant Major Dog Tag - Reach Rank Sergeant Major
Sergeant Major 1 Star Dog Tag - Reach Rank Sergeant Major 1 Star
Sergeant Major 2 Star Dog Tag - Reach Rank Sergeant Major 2 Star
Warrant Officer One Dog Tag - Reach Rank Warrant Officer One
Chief Warrant Officer Two Dog Tag - Reach Rank Chief Warrant Officer Two
Chief Warrant Officer Three Dog Tag - Reach Rank Chief Warrant Officer Three
Chief Warrant Officer Four Dog Tag - Reach Rank Chief Warrant Officer Four
Chief Warrant Officer Five Dog Tag - Reach Rank Chief Warrant Officer Five
Second Lieutenant Dog Tag - Reach Rank Second Lieutenant
First Lieutenant Dog Tag - Reach Rank First Lieutenant
Captain Dog Tag - Reach Rank Captain
Major Dog Tag - Reach Rank Major
Lieutenant Colonel Dog Tag - Reach Rank Lieutenant Colonel
Assault Rifle Dog Tag - Obtain Assault Rifle Efficiency Ribbon 10 Times
Carbine Dog Tag - Obtain Carbine Efficiency Ribbon 10 Times
Light Machine Gun Dog Tag - Obtain Light Machine Gun Efficiency Ribbon 10 Times
Sniper Rifle Dog Tag - Obtain Sniper Rifle Efficiency Ribbon 10 Times
Shotgun Dog Tag - Obtain Shotgun Ribbon 10 Times
Pdw Dog Tag - Obtain Pdw Ribbon 10 Times
Handgun Dog Tag - Obtain Handgun Ribbon 10 Times
Melee Dog Tag - Obtain Melee Ribbon 10 Times
Resupply Dog Tag - Obtain Resupply Efficiency..