Some 3 year old children still wear diapers full time and are not potty trained which is normal.
Usually children are potty trained before age 3 but some children take longer to potty train than other children and may stay in diapers past age 3.
Once the child gets to near 4 years old then they really need to be potty trained because that is getting too old to not be toilet trained and still be wearing diapers during the day unless they have an actual need to wear diapers.
A lot of 3 year old children will still be wearing diapers but some may not so yes some 3 year olds wear diapers all the time while others do not.
Some 3 year olds may be potty trained during the day and wear underwear during the day and then wear diapers only at night.
My son wore diapers at age 3 during the day and night but potty trained just before he turned 4 years old and he decided he wanted to use the toilet on his own when he decided that he didn't want to wear diapers anymore.