All babies are different and can weigh different amounts at different ages but the average normal weight for a 3 month old baby is around 13.2 lbs but they might weigh a bit less or a bit more than that.
A friend had a 3 month old baby who was pretty chubby and weighed 14 lbs which was a bit overweight which I believed but the doctor said it was normal for him due to his parents weight.
But as long as you're baby is around 13 lbs at 3 months the baby is at a healthy normal weight but being a bit heavier in weight then that is sometimes normal as well.
When the baby gets to 4 months of age they generally weigh around 14.8 lbs and when babies are first born they can weigh anywhere from 5 lbs to 7 and even 8 lbs of weight.