The weight range for size 7 diapers is 41 lbs and above.
Most 3 year olds are wearing size 6 diapers but some 3 year olds may need to wear a size 7 diaper if they are larger in weight than others.
However size 7 diapers would fit 4 years old and above and could likely even fit a 7 or 8 year old child if they are incontinent or need the size 7 diapers for bed wetting.
After a child outgrows size 7 diapers then you'll need to switch the child to youth diapers if they need to wear diapers.
My nephew who is 8 years old fits in size 7 diapers and he wears Pampers cruisers size 7 for bed wetting and when he's on a long road trip with his parents in case he needs to pee and can't make it to the restroom or if he falls asleep and wets his pants.
So his mom has him wear size 7 pampers cruisers on road trips just in case and he always wears the pampers cruisers at home for bed time for bed wetting.
They keep him dry and he doesn't wake up in a wet bed or wet clothes as a result.