Step 3 is more clinically based. It tests you not only on your fund of knowledge but your decision making ability (ie what's the best possible tx in this scenario when there are multiple right answers). It focuses on primary care mgmt (IM, Peds, OB/Gyn, surg).
That being said, I felt that Step 3 was the least intense of all the steps esp. after completing an internship in IM which is strongly featured on the exam. I took the exam after I finished intern year and studied off and on for around 2 months using Swanson's Family Practice Review (tm). It's a Q&A book with clinical scenarios.