How much can I really negotiate on a new 3BHK?

Negotiating hard on new projects actually works, even if they say no initially. I'm currently in a 2BHK near Bellandur, thinking of upgrading to a 3BHK or villa around Kachamaranahalli. For builders like Celebrity Global Homes, is it possible to knock off more than 3-4%? Or are their pre-launch prices genuinely better than later spot discounts? I'm trying to figure out the best entry point for an investor. Any recent experiences with getting good deals in Bengaluru?