The average weight for a three-and-a-half-month old pit bull is between 15 and 35 pounds, according to PuppyChart. Averages vary since every puppy is different and grows at a different rate.
If the puppy falls outside of those averages, pay attention to the growth curve. It's more important that the puppy maintains steady development.
ModernPuppies suggests it's helpful to know the weight of the puppy's parents in order to properly estimate how much he will weigh when fully grown. The average weight of a full grown pit bull is between 57 and 67 pounds for both males and females. Keep in mind that mixed breeds in the parentage will affect the rate of his growth and development.