adding node to existing availability group Currently have a 2 node availability group, trying to add a 3rd.

Added Node3 to the Windows Cluster. Node3 appears as UP in "Nodes" on the Failover Cluster Manager console. However I have the following message on Cluster Events:


  
    Cluster network name resource 'CAUBC-CLamf' encountered an error enabling the network name on this node. The reason for the failure
    was:   'Unable to obtain a logon token'.
  
  
  The error code was '1326'. 
  
  You may take the network name resource offline and online again to
  retry.


Notably, the Cluster Core Resources console doesn't have the IP address for the new cluster node. 

Another source of confusion here is found in the cluster log file:


  Logon failed for user CAUBC-CLamf$


Where is this user coming from?

EDIT: despite this error, I have been able to add the new node to the availability group and automatically seed the databases. I don't get why the error persists, however.

In addition, only the new node's ip address has registered in DNS - the listener/cluster resource ips are not in DNS

Is it necessary to add the new node's secondary ip to the Cluster Core Resources, in a multi-subnet setup?
I think this is normal behavior as only one cluster IP will be up at one time. When trying to force the secondary or third IP up you should get the error message you stated.