Hi,
You need to nat the remote lan - 192.168.0.0/16 on each respective sites to different subnet so that these appear as unique ip on the srx. And match the similar proxy on the srx.
For example you nat the 192.168.0.0/16 to 172.18.0.0/16 on SiteA and similar the on the other sides.
And match 172.18.0.0/16 on the srx as remote proxy id. leave the srx lan as it is.
Either policy based or route based will work. IN route based vpn, you specifiy the natted network in the route, and in policy based you specify the natted network on the firewall policy!
hope this helps.
rewanta