Some 3 year old children wear diapers and are not potty trained and some children are potty trained before 3 years old and don't need diapers.
So yes some 3 year olds need to wear diapers if they are not potty trained while other 3 year olds are fully potty trained and don't wear diapers.
Some 3 year olds may be potty trained in the day and don't wear diapers in the day but will need to wear diapers at night for bed wetting.
I was not potty trained myself until around 4 so I wore diapers at age 3 and I needed diapers at age 3 because I was a stubborn 3 year old who refused to use the toilet so I had to wear diapers until 4 years old when I decided to use the toilet.