Not with PIX, which doesn't allow packets to leave an interface they came in on. If PIX A and C each have a tunnel to B, for A to talk to C through those existing tunnels, A would send the packet to B's outside interface (assuming all the crypto map commans are enabled on the outside interface), then B would have to send it out the outside interface which is just came in on = not going to happen.
Even if it did work, it would just add more latency and network utilization (because everything from a to c would have to travel in and out of B), so it would be worth avoiding
