This is actually solved, the issue was that the actual link of the other two hosts was a dedicated network that wasn't reachable for node 3. I added additional links to the corosync.conf of the existing cluster, afterwards it worked
Due to a migration to a different datacenter, I want to add an additional host to the cluster. It's connected via site2site VPN, but in another subnet.
Firewall is opened, the hosts can ping, ssh, etc. each other. Unfortunately adding the host...