To potty train a 3 year old who refuses you have to force the issue.
You have to remove the diapers and make them unavailable for the 3 year old to wear.
Then put the 3 year old in underwear and he or she will pee or poop in the underwear and then make them clean the mess up.
Don't scold the child for messing in their pants or peeing in their pants but instead just tell them that it's okay and that they will do better next time.
The child will eventually decide that going to the toilet feels better than pooping in underwear or peeing in underwear.
If the child has no more diapers then they will have to use the toilet which they will do eventually even if it takes a few weeks or a few months.
However if the 3 year old is really not ready then the best thing is to put them back in diapers and keep them in diapers and try again 3 to 6 months later.
Eventually the 3 year old will potty train.
When a child is over the age of 3 years old and is not potty trained then it's considered late for potty training.
However I personally wouldn't consider it late for potty training until the child is 4 years old or older.
Some kids take longer to potty train than others and although some kids can potty train as early as 18 months to 2 years old and even 3 years old it does not mean that every child is ready for using the toilet.
Also a child really has not much control of their bladder until they are at least 4 years old.
So while a child may be able to physically use the toilet by age 3 years old the child's bladder cannot really hold it long enough for the child to make it to the toilet before they wet their pants.
This is one reason why kids below 4 years of age have pee accidents in their pants when potty training as they cannot hold the pee too long.
So it's best to wait until after 3 years old to potty train a child unless they are ready.
I've known kids who didn't potty train until age 5 years old even though they had no health issues.
They just were not ready and it's not worth the frustration to put up with to get a kid toilet trained and out of diapers.
A child will potty train when they are ready and I would rather change a diaper instead of dealing with the frustration of potty training a child who refuses and is clearly not ready.