you can't take it until after you start internship. it would be nice to take it in med school immediately after step 2 while the material is still fresh, but the step 3 application requires signatures from people at your internship program. they're not going to sign it if you're not working there yet. the application requirements also specifically say you can't take it until you've graduated from med school.
during internship year, the earlier you take it the better. in march, everyone is taking it and many end up having to take it far away from home b/c the centers are booked. there is also a deadline for some programs to take it by march or april or something like that.
if you have an ER rotation early in your internship year, do that first b4 taking step 3. the ER month is extremely high yield for step 3.
i just took step 3 (after finishing the first 4 months of internship). it wasn't that bad, and i didn't study at all.