3 inches is just right, and knit the foldover portion about half an inch longer than you want the finished cuff to be, since some of the length will be stretched out when you put the sock on. It has to stretch to fit your calf, and the stretch will come from the length. Also, if your pattern says, after you complete the cuff, to turn the sock inside out and continue in pattern, you can disregard that and keep up your K4P2 rib. When you fold over the cuff, it will show K2P4, which I like.