..just heads up on the help-map that is provided on the doc cd.
http://www.cisco.com/en/US/docs/ios/ipmulti/configuration/guide/imc_inter_mc_helper_ps6350_TSD_Products_Configuration_Guide_Chapter.html#wp1056657
its a nice reference but remember to flip the logic from the above example because you are taking multicast in on R3 (and not broadcast like example)..and sending out broadcast to R2.
..old forum below..
http://forum.internetworkexpert.com/ubbthreads.php/ubb/showflat/Number/23175/page/2#Post23175