I cant comment on BF3 as I didn't play it as I dont like the game play/style.
As for MW3, it has now gone back in the drawer for a while as it is really pissing me off and I will end up smashing my iPhone,MBP etc etc in a fit of rage lol!
There are definitely issues with it as its plastered all over YT, you only have to watch a handful of videos made by the likes of Sandy Ravage and Chris Smooth etc to see they are having issues with this game i.e the lag.
As far as my connection is concerned, I am with VM on 100Mb Broadband with very good pings, minimal jitter, no packet loss etc etc so as far I as am concerned I have a very good connection, if not one of the best consumer connections available in the UK!
The players I have issues with i.e lag, hit detection are all from the UK so I personally believe that the game is just concentrating on putting players of close proximity together in lobby's regardless of whether they have a good connection or not, but it does seem that certain people dont have issues so its maybe those players that have a "mediocre" connection and are "getting away with it" so to speak!
What makes me laugh as well is when I search for a game, it always finds me games that are <50 ping, so the game must see that I have a "good" connection but it's also seeing other players has having a "good" connection but then when the game starts that isn't the case i.e host migration, unbelievable lag etc!
It was fine a few weeks ago, but they have released 2 more patches that have made it worse imo, but it doesn't seem that I am alone!