MW3 and BF3 are completely different, the multiplayer alone proves that - MW3 you kill each other as troops, i've seen you can get a perk for a delta force to come onto the map and fight with you, and a full riot shield squad too, usual hard ass perks like heli and jets to come onto the map and help you out but for the most part your on your own.
On BF3 you can destroy cover, drive jeeps, tanks, jets, heli's and the whole point of it is to team up with each other and take on the other side.
How they handle is completely different, with MW3 when you fire you get recoil and the gun target tends to just jump slightly upwards when firing anymore than a burst, on BF games the recoil makes the target jump all around, from left to right and up and down.