Dice’s Executive Producer Patrick Bach has made it very clear that although Battlefield 3 is very competitive in the FPS genre it is not a Modern Warfare 3 copy.

Bach told Gamespot;

Were trying to build the best Battlefield game that weve ever built,
so it doesnt really change how people want to position it, and how
consumers see it or how media likes to portray this,

He was asked about the way EA is marketing Battlefield 3 as a direct competitor to Modern Warfare 3,

“To us its important to stay focus on building a Battlefield 3. We
dont want to play second fiddle or be a copy [of Modern Warfare 3] no
one wants to be the cover band, you want to be the star, so trying to copy someone else will also put you in second place. We
know that we can build great Battlefield games and thats what were
doing here as well.

Source: Gamespot