I tried adding the t3:// prefix and it still doesn't work.
I was previously specifying in the web console:
Number Of Servers In Cluster Address: 3
Number of servers to be listed from this cluster when generating a cluster address automatically. This setting has no effect if Cluster Address is explicitly set.
It is not necessary or the right practice to provide the protocol in front of WebLogic Cluster address.
It must look like ip1:port, ip2:port, ip3:port...
I suggest you to enable the cluster under debug tab of all cluster members (participating managed servers), run the test and look for specific messages related to replication. If you require assistance after that, you can post the log snippet here.