Problem solved (re-post, in case the preb post was not approved)

[Geroge Catalin]

Long story short (I don't know why the previous post went into pending approval state) , the issue was the group replication conf option : "group_replication_ip_whitelist" .

Don't forget to set that on all your nodes if don't want the plugin to block everything except node subnet + localhost.

My 3rd node was in a different subnet than my primary + 2nd node, so the primary was blocking the 3rd node subnet from joining (because the whitelist was set to default AUTOMATIC).