Yes the 3 year old should wear diapers if they are not potty trained yet or they need to wear the diapers at night for bed wetting.
However once the child is getting to 3 years old and beyond it's time to look into potty training and most children around 3 years old are ready to use the potty so it's worth a try.
However if the 3 year old refuses to use the toilet then just back off and wait a few more months and try again.
It's usually not an issue until the child is 4 years old and above and not potty trained and then you need to really force the issue.
A child cannot attend kindergarten wearing diapers and not potty trained so you need to have the child out of diapers before they go to kindergarten.
But at 3 years old I wouldn't be too concerned with the child not being potty trained and they should wear diapers if they are not potty trained.