While I didn't experience this myself, I have 5 friends who were all told they were having enormous babies (at 32wk scans) and none of them ended up having a baby over 8lb. From what I've heard it's quite difficult for them to tell exact size in the third trimester. Did they specifically say that your baby is measuring bigger than 28wks (sorry, I'm not sure if 3lb is big)? I know it's no comfort right now but I'm sure if your Dr is worried they'll speak with you about inducing you early. Best of luck :)