When my son was 3 years old and still wearing diapers full time I would change his diaper every 3 hours to 4 hours unless he pooped.
At 3 years old they tend to pee less often and sometimes hold in their urine and then pee all at once every 1 hour or so.
With really absorbent diapers such as Pampers disposable diapers they absorb the pee so that the kid does not feel wet unless they are soaked.
I would let the 3 year old wear the same diaper until it got full enough to need changing which was either after 3 hours or after 4 hours.
Of course at night the 3 year old would remain in the same diaper all night and be just fine.
Just check the 3 year olds diaper every 1 to 2 hours when possible and if it's squishy enough then it's time to change it.
My 3 year old went through around 4 to 5 diapers per day.