The size diaper that a 3 year old child would wear is either size 5 or size 6 diapers depending on how much the 3 year old child weighs and how big they are.
Some 3 year olds weigh more than other 3 year olds and some may be a bit fatter than other 3 year olds or may be a bit taller than other 3 year olds.
So not all 3 year olds wear the same size diaper or same size clothes.
But usually a 3 year old will wear a size 5 or size 6 diaper and some 3 year olds might wear a size 7 diaper.
Size 6 is usually a big enough diaper size for most 3 year olds to wear.
My 3 year old son wore a size 6 diaper which was big enough for him.