What does it mean for a dentist to say you have a class 3 malocclusion?

Prognathism. or an Angle Class III malocclusion means that the lower jaw protrudes forward and has a significant genetic component.
Cl III Malocclusion. Malocclusion means "bad bite." Class II means upper teeth and/or jaw too far back, lower teeth and/or jaw too far forward, or any combination of the above. Ask your Dentist to refer you to a fully qualified Orthodontic Specialist for an Initial (usually free) Examination. The Orthodontist can then best advise you as to how to best resolve your malocclusion.
Class III. Simply put, the lower teeth are more forward in comparison to the maxillary teeth. This can result in an anterior crossbite (underbite).