Used to be pretty common before the age of antibiotics. Was once one of the most common causes of insanity.
This stage is when neurosyphilis emerges and can cause any number of neurological problems ranging from nerve palsies, epilepsy, confusion, tremor, paralysis, parasthesiae to poor judgement and insight, tabes dorsalis or just plain going insane.
Cardiovascular syphilis can cause aortitis, aortic aneurysms, aortic valvulitis and stenosis of the coronary ostia, although involvement of smaller blood vessels such as the carotid artery and subclavian artery have been reported.
The classic sign of tertiary syphilis is gummatous syphilis. Gummas are granulomatous, ulcerating lesions which may appear on skin or mucous membranes and in bones and viscera. Skin lesions can appear anywhere but tend to prefer sites of trauma. Bone involvement usually involves the skull, tibia, fibula and clavicle although any bone can be involved. Visceral gummas usually involves the liver and the testes.
Treatment, as for normal syphilis, is with penicillin.