The ETS website says "at least one trained rater" scores an essay and their e-rater (an automated scoring system) scores it after. If both scores are close, the score you received is the average of the two. If they aren't close, a second person grades it, and the score you received is the average of the two human scores.
I haven't done a score review before, but I did use their ScoreItNow! writing service that utilizes their e-rater to grade your practice essays. 2 of my practice essays were the same score as what I received on the actual GRE and the 3rd essay was 1 point below my actual score. If you feel pretty confident that you did better than a 3, I don't think it would hurt to do a score review. But from my experience, their e-rater seems to be a decent grader of your essays, so I'm not sure if you would see a significant improvement in your score if you do go through with it.