Is it worth buying Modern Warfare 3? I just had a look at the server list, pff, it's only like few players playing zombie

If you want to continue the single player story, yes. About the multiplayer, I don't think it's being played as much as COD4 for some reason. Probably due to the lack of an easy crack to allow pirated game clients into the server.

The peak amount of players for the past thirty days is about 2,364.

Reference; http://steamcharts.com/app/42690

During the Steam sales, you'd want to purchase the game within a bundle or as a single title.

The next Steam major sale is the summer sale:

www.whenisthenextsteamsale.com

It's up to the developer(s) of the game to participate in the sales and they've done so over the years and i don't think they'll digress from this during this year.

Reviews of the game; http://steamcommunity.com/app/42680/reviews/?browsefilter=toprated&snr=1_5_reviews_