Yep. The single-player campaign is available for everyone, but the people who pre-ordered got to play it a week early.

The shortness of the campaign could be a contributed to a couple factors: there were rumors that MW3 started off life as DLC for MW2; the game developer Sledgehammer Games had come out and said they were initially tasked with developing Advanced Warfare 2 (I think) but that project was cancelled and they pivoted to MW3 and got that completed in 18 months.

Either way, the campaign is a big disappointment. Multiplayer seems to be getting enjoyed by a lot of people, though. Haters gonna hate.