Hello Ajay,
This may have gotten more response in "New Introductions" as u posted specificially in the D5100 section.. Having said that, let me say Welcome!!!
You have a asked a tough question... What are your intended photo subjects (people, candids, portraits, landcapes, buildings, pets, wildlife??? Still or Video?? How much time do you want to put in to learning a system or will you be putting it into auto and scene mode?? And the big one, how much are you looking to spend (in other words, your budget) (Keep in mind, you will need lenses based on this same criteria.
On your short list, they were all available when I bought my D5100... (for the record, I now also own the D7100)
I chose the D5100 as I felt it was more to what I needed then the 3100/3200 series and it was actually cheaper than the 3200 which had just been released. In spite of the 24mp sensor on the 3200, in my mind the 5100 was what I needed to start. I had done extensive comparisons between it and the canon, in that model, the Nikon was a slight plus for still image, the canon was a plus for video.. since I was not doing video, I went with the nikon.
I looked hard at the D7000 at the time, really liked it but felt it was too much more money and I did not know how serious I would get with this hobby.. end result, got the 5100 and was very pleased...
There is a 5200 out since I bought and you can get some deals used or refurb on the 7000 if you wish to go that way.
Good luck, figure what you want to shoot and which offeres the best features for your needs... DO go in and handle them to see how they fit you and how comfortable they are in your hand.
Pat in NH