Why was the the 3rd season shorter than the others???

The third season was shorter due to the writer's strike.

As the others have said, yes it was the writers strike that caused it. Initially, they only had 12 episodes before the strike commenced, so the season was going to finish then, however, after the strike ended, they managed to get permission/time to shoot 4 more episodes, leaving the third season with 16 episodes.

Cuz the writers got a burr up their a$$es.

The way I see it; seasons are like paragraphs. When the subject ends you start a new paragraph. So the end of a season means the start of a new story.