Hi, I understood your problem. But according to me, you have to make an owners association , if you have secretary in your apartment , summon him to your association meeting. Discuss with him about above said matters. Otherwise check in your brochure & sale deed paper about the amenities & of constructing 1 more tower. If nothing like this has mentioned then you can take legal action as per law.
As per my knowledge, better you consult with local advocate about this matter. Builder has no rights to threaten any of flat owners. As Pravallika said you form an owners group , discuss among each other. Send legal notice to builder not as individual but as an association. Then see what happens .