Week 3 is a good time to start them on nutes but introduce them gradually - 1/4 strength at first, then up to 1/2 strength and finally up to full... Monitor them carefully, though! I did this with 2 grows I'm doing now (Glueberry Auto and Lemon Auto) and nobody liked the nutes at first... it was TERRIBLY apparent - so I had to do a flush and start over with my feeding program... Each plant, each strain is a little different and they will tell you how you're doing - just pay attention to what they're saying!
Happy growing!