You should take Step 3 when you feel most comfortable with it; even if you score somewhat worse than what would be expected based on 1 and 2, this shouldn't have too much of a bearing on your rankability (as long as you don't fail it, or just barely pass).
Also, have you done any Radiology rotations? You mentioned that Rads was one of your top interests, but I'm not sure how far you got with it before finally settling on OBGYN. Remember, that just because you didn't like your PGY-1 year of OBGYN doesn't mean the rest of residency or your entire career would have you feeling that way. It's just so easy to find yourself in a "grass is greener on the other side of the street"; Radiology is actually one of the most common "switcher" destinations (next to IM and FM) for people who find that they're unhappy in their matched specialty and are looking for something more lifestyle friendly. If you decide that this is what you really want, you're going to have to work fairly hard to demonstrate your desire to train as a Radiologist, including securing LoRs from Radiologists. On the interview trail, IMHO, there's a fairly good chance you'll be hearing the question "You thought you'd be happy in xxxx, but you were mistaken; so what makes you think you'll be happy in Radiology?" Your answer to such a question would have a tremendous bearing on where you end up on that program's rank list compared to MS-4s who picked Rads first.