Many 3 year olds are not completely ready to use the toilet even when they can physically hold it.
In reality a 3 year olds bladder is still developing and still can have wetting accidents.
While a 3 year old can use the toilet and have some bladder control it doesn't mean they will make it in time to use the toilet.
A child should wear diapers until they are both mentally and physically ready to use the toilet.
A child that is ready to use the toilet and stop wearing diapers should be able to go to the toilet on their own, pull their pants down and pull them up on their own without any assistance.
If you're having to remind the child to go to the toilet and help them flush and wipe and pull up and down their pants then they really are not toilet trained.
Also some 3 year olds and older kids may need diapers at night for bed wetting which is normal even when they can use the toilet at night.
Diapers are a good option for 3 year olds that wet the bed or any other kid of any age that wet the bed to keep their clothes and bed dry and help them get a good nights sleep.