how id set it up is (not including satellite feeds here)
first seperatly flash an image onto the 3 boxes
turn off DCHP in the network settings and assaign 3 fixed ones (101, 2 & 3 for example)
run a single cat5 and plug it into a cheap 5 way network hub, then a small cat5 patch lead to each of the 3 boxes
setup the scart-RF modulators (DONT buy cheap ones of these m8, this is where the main weak link will be if you do) with a gap of 5 channels between them
instead of looping through the modulators, id use a 3 way tap in reverse, as this will also act as an RF equaliser and reduse and noise or cross mod introduced by the other modulators
next, using one remote, id download a cam onto all 3 boxes at once, and the config file
now id either download the remote lock to each box, or tape 2 of them up, and edit the configs using a laptop ready for the cards
once done, put the cards in, restart all 3
setup a startup service channel on each box
restart all 3 again
and enjoy true multiscreen viewing
as itll all be on an internal network, no port forwarding will be needed,