Thanks, I saw the link provided above too. That scares me, it seems we have to uninstall SQL server totally, it will be a long process , esp. for a database server with many sharepoint databases.
Glad to see sandtoad2's answer that after SQL server 2008, we no longer need to uninstall the whole SQL server program, but just uninstall the service pack, this makes me feel much better.
I just want to get prepared if a failure happens, I can rewind, roll back to previous SP.
Basically we want to do SP3 for a SQL 2008 SP1 sharepoint database cluster.