To get your three year old out of diapers you should put the 3 year old in underwear and keep taking them to the toilet when you notice they are about to pee or poop.
Get to know the child's pee and poop schedules if you don't already and set a timer to those times and when your child is about to go pee or poop take them to the toilet and sit them on the toilet.
Give them praise when they do use the toilet but don't punish them for accidents.
With underwear the child will feel wet and not like the wet underwear or poop underwear and will eventually decide to use the toilet.
However it's normal for 3 year olds to wear diapers and not be potty trained so if they still are refusing to use the toilet then back off for a few months and try again.
Your child should ideally be out of diapers in the daytime by age 3 to 4 years old.
However every kid is different and some kids are still in diapers until age 5 years old which is okay.
A child will eventually potty train when they are ready even if you don't try potty training them.
Although it's still a good idea to introduce your child to the toilet by age 2 to 3 years old so they get used to the idea but don't push the issue too much.
There's no set age that your child should be out of diapers in the day.
However it's best to try to get your child potty trained starting around age 2 or 3 years old.
Most kids will eventually just decide to stop wearing diapers and go to the toilet.
It's not worth the fight to try and force a child to use the toilet when they really are not ready.
Some kids are ready to potty train and be out of diapers by age 18 months while some may not be ready until age 2 or even 3 years old.
In rare cases a child may not be potty trained until 4 or even 5 years of age which is okay as well.
Diapers are so absorbent now that the child does not feel wet and they are more comfortable in the diaper even when wet or dirty.
I would not start worrying about potty training your kid until they are 5 and still not using the toilet.