The three students including a girl were studying in medical colleges near here.
Earlier, the police had arrested student K. V. Udit Surya and his doctor father Venkatesan, working in the Government Stanley Hospital in Tirupati and an agent in Kerala for hiring a proxy to write the entrance exam.
The scam is being probed by the Crime Branch Crime Investigation Department (CBCID) of Tamil Nadu police.
While interrogating Surya and his father Venkatesan, police learned that there were several other first year medical college students who had engaged proxies for a huge fee - upwards of Rs 20 lakh - to write the exams.
Surya was studying in the Government Medical College in Theni in Tamil Nadu.
According to police, Praveen of SRM Medical College, Abirami of Sai Satya Medical College and Rahul of Balaji Medical College have been detained for inquiry.
The proxies of the four students wrote the exams in Delhi, Uttar Pradesh and Mumbai.
Police said the original photos in the hall ticket were substituted with that of the proxies while all other details remained the same.
The scam came to light when the Dean of the Government Medical College in Theni got two emails from a person, Ashok Krishnan, complaining against Surya.
According to the complaint, Surya had failed twice in the NEET. He had cleared the recent exam opting for Mumbai as the exam centre, where a proxy wrote the entrance exam.
The college authorities found two different photographs.
Meanwhile Venkatesn has been suspended from service.
( With inputs from IANS )