Depends on your school/opportunities/ability to juggle/time commitment of TAship.
I know a fair amount of medical students that have TA'ed undergrad classes at my school. It's also incredibly lucrative (i.e. it basically will cover most of tuition and pay you a healthy living stipend). This is almost certainly not during third year. But, a lot of second year students and fourth year students do.
I've tended to avoid TAing undergrad classes because they're actually a lot of work (grading, review sessions, etc.)...upwards of 15-20 hrs/week. I've TA'ed 2 medical school courses, and they're nice because everyone generally understands that you aren't going to be devoting that sort of time to it, and while the "advertised" commitments are usually 10 hours, most only required 5-7 hrs or less per week. Which, to be honest, I think is doable as a second or fourth year student if you're looking to make some $$$. Plus, they also give you some teaching credentials that might look impressive to residency directors.