When should Step 3 be taken so those without an American citizenship or a green card can be eligible for a H1b visa? Would it be difficult passing Step 3 without going through your intern year?
1. As soon as possible
2. It is a standardized test. While it is helpful to have clinical experience, with dedicated studying it is entirely possible to pass it before internship.
Only a few states allow you to take it without being enrolled in a residency and without applying for a license at the same. I believe CT, FL and MD are the most popular states from this aspect. The result will be valid in all states, no matter what state board you used to register.