I assume that router A is connected to router B and router router B is connected to router C. If this is the set up then you are able to ping A to B and B to C because they are on the same network. For A to ping C there needs to be a route. This can be done by adding a static route on router A and C or configuring a routing protocol on all routers.
Here's a good article that explains how static routes are configured Cisco routers.
Last Wiki Answer Submitted: December 29, 2009 3:33 pm by Dave Bateman240 pts.