Yes.... on the inactive node stop all unneeded services like backup software, AV, etc.... run the upgrade to SP3 and it will update the binaries and will run rather quickly.
When you are ready fail over SQL Server to that server.... it will then upgrade the databases to SP3 by running some scripts within SQL Server.... wait a few minutes and connect to SQL Server and look through the log to make sure it all worked.
Then, simply run SP3 on that now inactive node.... once all done fail SQL Server back over to that node to verify failover works. Look at logs and verify all is OK.... YOU ARE DONE>
Make sure you take db backups before all of this.