Umm..for agni-3 to cover whole of CHina you will have to launch it from NE to just make it to beijing and eastern china.Of course,I hope you can understand that stationing your nukes so close to the opponent is a downright retarded idea,especially if using missile silos which I don't think India does or at least not much or even in mobile launch platforms the pitiful infrastructure will hit feasibility hard.Er....Agni 3 already covers all of China. Plus Agni-2 alone covers major Chinese cities. In a nuclear war with there is no point in targetting every nook and corner of China, unlike India China has 50% of its population in cities
There are very good reasons why nuclear big boys see boomers and SLBMs as the most desirable of the nuclear system.