The way that Step 3 works is that based on the state that you are trying to get licensed in (i.e. where you are doing your residency), there are stipulations on when you can sit for Step 3. Some allow you to take it immediately upon graduating med school, some make you wait a month and some make you complete internship first. My advice to you, no matter when you are eligible to take it is that you take it as early as possible. You will be surprised how quickly you start to forget psych, OB/gyn, peds, etc. Just take it and get it out of the way. And remember, all you need to do is pass, so don't study too hard. It is exactly like Step 2CK but with an extra day semi-devoted to clinical scenarios which are cake as long as you go through the practice questions on the USMLE CD that you get with registration for the test.
Good luck.