If I understand your description correctly what you have is a classic triangle connected by T1 point to point (equal cost paths). The classic solution if to route each city to its neighbor directly and perhaps to use the indirect path as a failover mechanism in case of failure of the direct connection.
What you indicate you want is to load share between city 1 and city 3 between the direct connection and the indirect connection. You do not indicate whether you need the same capability for city 2.
I assume that you currently have a static route at city 1 identifying the network(s) at city 3 with the route pointed at the T1 direct connection. An easy implementation would be to add a static route for the city 3 network(s) pointing to the city 2 link as a path. The router would then have 2 eaual cost paths and would load balance. This is simple and it would work, but one of the downsides of this implementation is that if there were a failure of the link between city 2 and city 3 the route at city 1 would continue to send some traffic for city 3 to city 2 but city 2 would not be able to forward it.
The solution to the problem mentioned above would be to run a dynamic routing protocol. The primary advantage of the dynamic routing protocol is that it becomes aware of changes in the remote parts of the network and makes appropriate adjustments. If you want to load balance this with a dynamic routing protocol using EIGRP would be the optimum solution because EIGRP is the only dynamic routing protocol that supports load balancing over unequal cost paths.
One other solution to consider might be the use of Policy Based Routing to achieve your purpose by directing certain types of traffic over the link through city 2. You would continnue to use the static routes that currently work for you which send traffic over the direct connection. You would identify certain types of traffic to send over the alternate path (you might choose certain protocols like FTP, or telnet, or anything else that makes sense in your environment, or you might choose certain source addresses or destination addresses) and configure PBR to send that traffic over the indirect link.
HTH
Rick
HTH
Rick
