@davidr #IPv4 or #IPv6? If you want to do it the real IP routing way (and if your IP multicast addresses are routable?) you can use things like #smcroute, #pim6sd or #mrouted. Many of these have been kept alive and/or got a revamp/resurrection thanks to @troglobit. I like/use pim6sd personally (with MLDv1 / any-source-multicast only so far, not source-specific-multicast).

@davidr @troglobit Of course, you could also kind of try to route things on your own, manually some layers above, with #socat for instance (though it broke for IPv6 in the recent version again, the dev has a patch, but don't know why he hasn't made a new release with it yet... v1.7.3.2 worked for me, nor has he added it to his Git either last time I checked).

@davidr @troglobit oki doki. Do you want to receive some content in an IPv4 UDP multicast to some 224.0.0.0-239.255.255.255 address on one host and forward it to a single (or few) specific other hosts by repackaging this to IP UDP unicast packet(s)? What does "on the same physical network" mean? (if all three were on the same, then the desired destination host would just be able to receive it on its own?)
I guess you have found this IP multicast socat page already?
http://www.dest-unreach.org/socat/doc/socat-multicast.html