Media for Cisco 837 between remote sites

Cisco 837 between remote sites

Hi , The secanrio is there are three remote office connected to internet all of them use cisco 837 router and 2 offices are working fine , one office the rounter is able to connect to internet but a connection is not established between sites, I had to do the rest, the router as it was installed by some one who had not given the password. My main job is now to connect the router , to the existing setup.

All i know is that i have a public ip ,

Router A : ip address 10.10.10.1 , public ip , a.b.c.d

Router b : ip address 192.168.4.21 connected to A ,

Router c has to be configured ,

How do i go about it ?

Thanks and Regards