You completed with:
At this point, all passive nodes has the service pack installed and it’s time to perform the update on the active node (serverA), so connect to it and perform the installation of the service pack. The process won’t be different at all as the one on the other nodes as we’ll update one instance at the time. Once the three instances were updated, the SQL Instances 1, 2 and 3 are at the same patch level on all nodes.
Did you fail over to another server first before upgrading the final server?
Give a man a fish, and you've given him a meal. Teach a man to fish and he'll ask to borrow your boat.:-P