Well, your mileage may vary but...
the transition back to 3rd year after 4 years in grad school was much, much less of a problem than I had anticipated. I had been dreading it for about a year, but it was actually great!
3rd year is fun! I had been catastrophizing 3rd year for a while, imaging a hellish year without any sleep and with nasty attendings constantly yelling and pimping me. I think that's actually what internship will be like, but we'll see...
It's actually not been like that at all in my experience. True, you have to get up early occasionally, and every now and then you come across a nasty resident/attending, but for the most part the people are nice, supportive, and (most important) interested in teaching. Plus, the stuff you learn is for the most part pretty cool.
The expectations that residents and faculty have for most 3rd years are actually pretty low--it's easy to surprise people. They expect students to know next to nothing, so when you demonstrate that you've been reading and are interested and helpful, you get nice evals.
In summary, my recipe for doing well in 3rd year is:
1) always show up on time.
2) look interested.
3) be pleasant and enthusiastic.
4) dress professionally.
5) read, read, read.
cheers,
-mrp