Is it only for back-up, and only for your own use? How about IPv6?
The problem for the second (and third…) Yunohost at home is the lack of IPv4 addresses at our home connection. Most home connections got an /64 or larger IPv6 subnet.
So, if you only enable IPv6 on the 3rd Yunohost, and configure only the AAAA in DNS for IPv6, there should be no problem.