Size 3 diapers fit babies between 16 lbs to 28 lbs and after 28 lbs the baby is usually ready to move up a diaper size to size 4 diapers.
But babies are built different and some may wear a size 4 diaper once they reach 28 lbs or before.
If the diaper is getting too tight around the babies waist and is leaving marks around the babies waist or legs then it's time to move up to the next size diaper.