I just finished M3 with across the board honors and my formula was pretty similar to those already posted here:
1) UWorld Step 2 CK
2) Pre-test for every rotation + some other similar source like Lange. I found the combo of 2 sources very helpful for covering the breadth of minutiae on shelf exams. I used pre-test every time not necessarily because it was the best, but because I found a bug in the iphone app which enabled me to get them all for free each rotation.
3) Didactics on each rotation were generally good and covered important exam topics
4) Additional readings as needed and usually covering highly important topics for that rotation. For example, you know medicine will ask you about CAD, DM, etc., so I would read on topics like that so there was no way they could ask me a question I couldn't answer.
More than anything else though, I found that learning from patients was most helpful, especially for figuring out the "next step in management" sort of questions. Of course, this also breeds confusion since the next step on the exam and the next step in real life are not often the same. Even so, this is the exception rather than the rule. I would encourage you to stretch yourself to pick up as many patients with as many different conditions as possible. Most conditions have a fairly predictable way of presenting and this is usually how they present on the shelf, so the more you see in real life, the better prepared you are. For every patient you see, come up with your own assessment and plan and then figure out why the ultimate plan is the same or different - great way to get a feel for next steps and tests to order. I was lucky that the residents on my first rotation (surg) forced me to do this and I just kept doing it throughout the year. Yes, I felt stupid a lot for coming up with a bad plan, but I got better.
Third year is a lot of fun, so enjoy it! That said, I'm still REALLY glad to be starting 4th year