Neuro-enhancement 3 is a symptom exclusive to the Simian Flu DLC. This allows to the virus to further increase human intelligence, increasing research speed. Coupling this with Blindness unlocks the Blind Genius symptom combo.
The difference from Neuro-enhancement 2, however, is that the virus will begin modifying neuron DNA to the point where there is a chance for the immune system to identify this as a threat and respond in an autoimmune fashion which causes the pathogen to be slightly more lethal.