I just took it before residency started... a couple months after pt2. So did a couple of my co-residents. Fyi to all, it's a 100% myth that you can't take NBPME pt3 until after your first year of residency. Completing at least one year of residency is a req to actually get your license in a lot of states, but not to apply. The only req for signing up for pt3 is that you have to have a full state license app in process (any state). NBPME will verify that you have an app in, and then you can register.
As for the exam, it was basically the exact same as pt2... mostly pod stuff. For any of the NBPMEs, just look at the content outline: it tells you all the topic areas tested. My surg knowledge was already more than sufficient since I'd just studied for clerkships + residency interviews. I studied PI manual (prob best resource for any boards in our specialty), Presby manual for medicine, and the NBPME Pearls book by Kushner for some of he oddball topics (biomech, pub health, radio, derm, etc).