Three is really two movies together.  The first half contains a genuine love story with aspects like love at first sight, parental slapping, the girl displaying more bravery than her boy, etc., all of which are handled in a traditional Tamil film manner.  Another movie, a psychological thriller, terrifies us in the second half as it explores the dangerously unstable parts of Dhanush's mind.  The movie 3 is primarily Dhanush's solo performance; the actor gives one of the most astute acting turns by a South Indian hero in recent memory.
Director Aishwarya's talent lies more in crafting a lovely romance drama than in terrifying or moving us to tears.  The first 20 minutes were like watching a movie.

Without Dhanush, the second half would have been a terrible experience because reason and intelligence had taken a vacation.  He performs with a tendency for perfection.  He doesn't exhibit any signs of being subpar and he doesn't mimic anyone else's acting style.  Watch him exhibit horror at finding the pet dead, bless his wife, and endure pain before taking his own life.

Shruti could not meet the criteria for an Intermediate student, but she was incredibly believable as a worried wife who was pleading frantically with her husband.  Prabhu was excellent as Ram's father, and Rohini was excellent as Janani's mother.

The conversations felt really natural.  Before her marriage, a remorseful Janani says to her father, "Nenu ippude chacchipotha nanna."  In theory, the musician (Anirudh's Kannuladha) owns the number 3.

Soon, the movie makes a radical turn.  Even though their parents do not accept their decision, Ram and Janani are married.

Janani, however, is not fully aware of Ram.  Ram has bipolar disorder.  The second half details Ram's harassing experiences leading up to his death because he was unable to cope with his severe mental illness.

There are various flaws with 3, for example.  The author's decision to expose Ram's demise early seemed a little reckless.  The (informed) audience would immediately understand that there is a psychic element because one knew that 3 was not a murder mystery.  Does the director want us to hold off on learning what the illness is? There was no justification for Sunder Ramu, Ram's friend, not to discuss Ram.

The best Dhanush film I've ever seen is this one. The film is jam-packed with romance, drama, and suspense. Dhanush is one of the few actors in Indian cinema who is equally at ease playing a school student, a businessman struggling with his inner demons (as he does in "3"), a young salesman looking for his bike (in "Polladhavan"), or an uneducated country bumpkin who only knows how to raise and fight roosters (in "Aadukalam," which earned him his first national acting award). He portrays Ram in both of his extremes without breaking a sweat, and his portrayal of the schoolboy in love is more enjoyable and watchable than the transformation into a married man in the second performance.