The Series 3 is the current model in B&W's lineup. Aside from used, it'll probably be difficult to purchase the older models as they have been discontinued.
Although, I can't say I am familiar of the Indian hi-fi marketplace.
The S3 incorporates some new design features- a tweeter derived from the Nautilus, revised enclosure and I think a revised crossover. There aren't huge differences but B&W has refined and evolved the DM 602 line throughout each successive generation.