Noctunal enuresis, or bed wetting, is a common occurrence at 3 years of age. Children first achieve daytime bladder control. The age at which bedwetting stops might be determined by genetics, so children may stop wetting the bed at night at the time their parents did.
Some children sleep extremely deeply and may not be able to feel the urge to urinate until they are much older, even teenagers! Again, that is determined by genetics in many cases.
Night time bed wetting should be a concern if a child achieves night time bladder control for at least 6 months, and then bedwetting resumes. It sounds that all your child needs right now is time and reassurance. If you are still concerned, talk with your son's health care provider.