In my experience, it was a lot more common for 3L's to be working part time clerking jobs. However, quite a few students also did work-study jobs. I tried clerking as a 3L but found it too time-consuming. Unless you're sure that your clerking work is going to dovetail into a real job after graduation, it's really not worth it. I got all the clerking experience I needed at a summer job in the second year and an externship.