What you will do with your laptop? Look at the screens, read some reviews about their quality, Everything else is the same (CPU, RAM, HDD, video)... And of course, look at what service / support you can get in your country.
I use it primarily for gaming and CAD modelling software like ANSYS. What is better? A 3rd generation i5 with a 1 GB graphics card or a 2nd generation i5 with 2 GB graphics, if everything else is the same?
notbookcheck.net is a pretty useful website.
The lenovo looks to be the better one, more than 1gb is a waste unless you are going to run multi displays or attach it to a 1080p display via hdmi, the 3rd gen i5 will use less power, and make you able to game for longer.
I'll say the Lenovo is the best one out of the three. 3rd gen processors are better than the 2nd gen ones. The difference between 7670m and 630m is negligible, I'll go for NVIDIA for the features.
Dell is good. I have my own -Apple MacBook . I just love it. I bought it from shopbychpoice @ Rs. 72,900/- (after 2% off)