thanks for your response.
In order to have site to site vpn for Site A and Site B, I have the following configuration. Because I am not familiar with Juniper, to have it extend to Site C. How can I add the route for Site C?
set vpn "Site_B" proxy-id local-ip 192.168.70.0/16 remote-ip 192.168.50.0/16 "ANY"
set policy id 3 from "Untrust" to "Trust" "SiteB" "192.168.70.0/16" "ANY" permit log count
set policy id 3
exit
set policy id 2 from "Trust" to "Untrust" "192.168.70.0/16" "SiteB" "ANY" permit log count
set policy id 2
exit
set route 192.168.50.0/16 interface tunnel.1