As a FMG you should be able to apply for step 3 after you get your ECFMG certificate AND graduate from medical school - you need your medical diploma in order to apply for step 3.
I think it would be a wise decision to sit for step 3 before starting residency. I'll start residency in July, and I sat for step 3 in the end of January (of course I needed it to apply for a H1B-visa - but I would have done the same if I had a J1-visa instead).
Point is that step 3 is an easy exam, you should have no problem passing it. I used 2 weeks of full time study and I passed without any problems...
Just go for it - good luck!
Edit: You can register in Connecticut, which will allow you to sit for the exam without having started residency. You don't need to take the exam in Connecticut though, you can take it at any Prometric center across the US. That's the way we IMG's sit for it...