VPN router to router issue

Problem Description: I have 3 sites (A, B, and C). VPN between A and B is already established. (A—-B—-C). Let say I want to implement VPN so C can access resources in A, but I don’t want to establish the tunnel between C and A. I want C to get to A thru B by router to router VPN. Can you give me instructions how to do it including detail docs? By the way, this vpn is based on site to site router to router vpn.