Some 3 year olds can be potty trained and are ready before then or at least at 3 years old.
However some kids are not ready to potty train until 3 years and 6 months to 4 years of age.
Some kids can begin potty training as early as 18 months while others take longer and don't potty train until near their 4th birthday or after 4 years old.
As long as the child is not wearing diapers during the day past 4 or at least before 5 years old then they are doing okay.
There's no set age that a child has to be potty trained but the child must be potty trained before going to preschool or kindergarten.
My son was not ready to potty train until he was 4 years old so he was in diapers full time at 3 years old and up until he turned 4 years old.
At 4 years old he decided that he wanted to stop wearing diapers and wanted to use the toilet instead.
There are a lot of 3 year old kids who wear diapers full time all day and night and are not potty trained and it's okay.
The child will eventually want to use the toilet.