I have been working on the same kind of setup. My best guess for you is to setup 1 bridge on each of your subnets. Set the <Bridge Configuration> just like you are doing on your last post (with their IP addresses) except when you get to
the <targets> add all the other bridges in there.
So for instance on the bridge configuration for 10.3.1.102 you would have:
<add ip="10.3.1.102" port="1236" />
<add ip="10.4.1.101" port="1236" />
<add ip="10.2.1.101" port="1236" />
Hope this helps.