3 years old is not too old to wear diapers.
Although some children are potty trained and out of diapers during the day by age 3 it's not a bad thing if your 3 year old is still not potty trained and still wearing diapers.
There's no set age that a child has to be potty trained at.
Actually potty training a child too early when they are not actually ready can cause health issues so it's best to allow the child to decide when to potty train.
Eventually the child will not want to wear the diaper anymore and will learn to use the toilet on their own.
Diapers are for all ages even if you're potty trained.
However a child should be using the toilet by age 4 at the latest unless they have autism or other health issues that prevent potty training.