Hello David
That’s a great observation. Indeed it seems that R3 should not have an mroute to 224.0.1.39 since mapping agents use this address to listen for candidate announcements. It makes sense for R2 to have it, but what about R3?
Well, it turns out that the mapping agent (R2) will send out IGMP membership reports for the 224.0.1.39 group as soon as the send-rp-discovery command is applied.
This is confirmed with the following command on R3:
R3#show ip igmp membership 224.0.1.39
Channel/Group Reporter Uptime Exp. Flags Interface
*,224.0.1.39 192.168.23.2 00:01:45 02:08 2A Gi0/2
If you enable debugs you will see the relevant membership report as well. This is why the group appears in the mroute table of R3, not because R3 received an RP announcement, but because R2 sent an IGMP membership report.
I hope this has been helpful!
Laz