My MW3 record is nothing right now, I've barely touched it but I have done decently in most of the CoD games.
Here’s my advice:
Know the map
You have to know the maps. If you don’t know the map you’re just in a discovery phase, kill/death ratio means nothing. You MUST be able to dash full speed and know the upcoming snipe points/hiding places/high ground nesting places, walled areas people like to shoot from behind. You must KNOW the map.
Know where your team is. Use that minimap to guess-timate what you don’t know from what you DO know. If your team tends to be congregated in one area of the map and none of them are firing you can anticipate where the other team is probably lurking. Try to flank that area, move with cover when possible as you approach the 'open' front. Follow a teammate’s lead into the fray, often they'll take the first bullet for you.
Never fight fair
Don’t engage the enemy unless you have the advantage. Avoid going toe-to-toe, just NEVER do that. Only kill when it’s a borderline dirty kill and you either totally have the drop on them or you’re able to fire on someone fully exposed from behind cover.
Don’t get into a face-to-face shootout with someone unless you’re tired of living. To be perfectly honest (with myself) most of the kids playing are way better than me, they can aim faster while moving. Usually if someone is willing to go mano-a-mano with me in the open it’s only because they feel they have an advantage and I don’t know what it is –– ergo - I should definitely take cover.
Use flash grenades for defence only. A lot of people try to use them as offence, I don’t make that mistake anymore. Yes, you get that targeting reticule the second your flash hits a target but you don’t know what the effect is, it could be minimal so diving in as if you got a sure kill is a mistake.
OTOH, using the flash to determine if someone is hiding somewhere within throwing distance (the targeting reticule goes off) is good to expose the enemy but don’t assume they’re incapacitated in any way. I like to use a flash grenade when I’m running for my life, leave one in my wake. So, if I’ve sniped someone and the cavalry is coming for revenge (I’ve seen two or more coming at me) I’ll switch weapons to something light, like a pistol and get the hell out of there, drop a flash right where I was sniping from as I leave.
When I’m hit by a flash with minimal effect I can’t wait to see the enemy come running around the corner thinking they’ve got me. It’s a zero sum gain, they know I’m there but I also know where they’re coming from now and they think I’m incapacitated by their flash. When I’m not – they’re usually dead.
Keep passion out of it
It’s only a game. A lot of people have a tendency to be incensed that they died… AGAIN so they run straight into exactly what just got them killed hoping for a different outcome. You’re frustrated (in a fun way of course) and you’re not thinking straight. When I get killed AGAIN and go “ARGH!” in frustration I go prone when I respawn. I just take two seconds to rethink what I’m about to do. If I spawned close to my death area then I am better able to deal with my killer from the prone position. Don’t just rush in… you’ll rack up deaths before you know it.
On the same token, play the way you play and don’t get drawn into a pissing match with other players. Everyone accuses others of being n00bs, campers etc when they die, it’s just the sign of a weak-minded individual. Don’t get sucked in and don’t let it take you off your game. Camping is a good strategy and any decent player can counter the camp especially when they’re aware of it.
Pick a build and be it
If you are new to the game stick to a lowest common denominator build… use the assault rifle and a grenade launcher. Yes, I’m calling it somewhat cheap, but it’s a legit build I find nothing wrong with it. It will help you in both tighter, smaller maps and large roomier maps. If you’re learning, go assault rifle/grenade launcher.
If you’re a sniper… 1/ pick up a secondary, drop that pistol. 2/ relocate after no more than two rounds unless your confidence is extremely high. I like to snipe personally, it’s my fav tactic.
There are obvious high-nest points. I don’t snipe from those. I take a low place in the grass next to the high roost that everyone is watching. If a hapless enemy sniper occupies that spot I’ll shoot him. Hiding in the open sometimes works – find unusual places from which to snipe, use angles to get wide views through doors, cave entrances and natural cover/barriers.
Don’t try to build a soldier that does too much. If you’re a sniper, be a sniper, don’t try to be a stealth sniper because it just makes you a less effective sniper. If you’re going to be a stealth character, go all in, use a silencers, avoid radar detection etc. etc. and be full out stealth.
Assault rifles with ACOG scopes are a half-assed sniper/assault rifle soldier IMHO which means they’re not as effective at either. Believe me, I’ve tried to bridge gaps with my builds. All things being equal, fighting someone of equal skill to you – they will kill you a majority of the time if you’re trying to be two builds at once. Pick a build and BE it!
Watch how just died. It’s a lost art but especially when you’re getting to know a map, you’ll find clever hiding spots you never thought of and learn interesting new tactics.
Watch your ping
If you’re below full four bars don’t expect a fair fight, you will not win a knife fight the enemy will beat you every time. If you have less than three bars don’t even play – find another game.