Hello,
hard to say without seeing the OSPF configurations of RHU and RS3, can you post those ?
Basically, on RS3 you could configure different OSPF costs for each tunnel (ip ospf cost), and on RHU, configure the 'default-information originate always metric' command to send the default route to RS3.
Either way, post the configurations of both routers, that will make it easier to figure this out...