In child-led methods of potty training, 3 is a fairly common age for children to express interest and have the ability to go potty by themselves.
Particularly for boys, 3 seems to be a sweet spot for potty training. Three-year-olds are often able to say when they need to go, pull pants up and down, sit safely on the potty, wash their hands, and take pride in their accomplishment. They may respond well to books and games that make it fun, as well as to rewards or charts.
Three-year-olds also may do particularly well with faster potty training methods (such as 1- or 3-day intensives) as they catch on quickly.
The typical age range for potty training is 18 months to 3 years old, with 27 months being the average age for potty training. So while many children do potty train at 2 years old, it is not unusual for a 3-year-old to not yet be potty trained. If you have a 3-year-old in diapers, this may be a great time to get started.