They never really gave us a solid guideline for that either.
The gist of it is that, as a 3rd year, no one will ever give you a hard time for showing up in your white coat and professional attire. When in doubt, do this. You can always put the coat over your chair and loosen your collar. I'd say more than half my class did this for every lecture/orientation/etc of 3rd year. It's definitely a must for grand rounds and anything where faculty will be present.
For orientation and lectures when you aren't expected to be at a clinical site, you might be able to get away with lighter dress. Personally, I'd test the waters and let someone else be the first one to go casual.