CCM locations are a star topology. Your topology is not. So C would have to connect back to the CCM Cluster directly.
You might be ok with locations A, B and C. Set A to none (if that's where you CCM cluster is), B to 5Mb (or lower) and C to 256. Phones and devices in C get assigned Location C. Might be OK since A to B is greater than A to C.
Otherwise probably have to use gatekeeper CAC. Put GK on a router at location B and C. Put bandwidth to 256Kb and ditch the CallManager locations CAC since you shouldn't (not exactly CAN'T, but shouldn't) use both GK and CCM CAC.