Step 1 is best taken immediately after the pre-clinical years. The minutiae that you're expected to know for the exam is quickly forgotten in the clinical years. Step 2 (both CS and CK) is best taken after at least a year of clinical exposure to the basic fields of medicine (Psych, Family, Peds, IM, OB/GYN, Neuro, And Surgery). If you could, I'd recommend getting clinical exposure in the US before taking either component of Step 2. And Step 3 can't be taken until after graduation and all other Step components have been taken and passed. It's also important to know that you'll have seven years to complete all components of the USMLE in many states, starting with your first sitting of Step 1.
