It is amazing how these 3 ethnic groups have so much in common.
I noticed 90% famous people known in the West and in the World that is Indian origin usually are Punjabis, Bengalis, Tamil (including nobel prizes, scientists, inventor actors and much more) excluding foreign ethnicity living in India such as Parsi people, Pashtuns people. Yes, go check the famous Indians, and you will see that they are usually one of the three ethnic group.
Those 3 groups are located in the corners of India (tamil in the very south, Bengali in the very East, Punjabi in the very West).
Punjabis, Bengalis and Tamil are all official language of India plus is somewhat official in another neighboring country (Bengali in Bangladesh, Punjabi in Pakistan, and Tamils in Sri Lanka).
Punjabis, Bengalis and Tamil people are mainly divided between two countries. (Punjabis are in Pakistan and India, Bengalis are in Bangladesh and India, and Tamils are in Sri Lanka and India).