This is step by step manual how to update SQL 2005 cluster to service pack 3. Microsoft SQL Server service pack are all cluster aware (which means they automaticaly detect clustered instances and update clustered features of the sql server) but anyway you need to know all the details when you are installing service pack to a sql cluster.

It is necessary to update each node of the cluster to get a SQL Cluster fully updated with all the tools and components to the latest Service Pack.
To avoid future errors during update process the user account under which SQL Server is running needs to be added to sysadmin server role.
Follow the next steps to update active node of your sql server 2005 clustered instance.
1. Launch SQL Server 2005 Service Pack 3 installlation package. After inicialization process is finished select all  the features you need to update (Picture 1). If it is not the active node of sql server cluster then Database Services and Analysis Services checkboxes will not be available.  Select all features you need and click “Next”

(Picture 1) - Microsoft SQL Server 2005 Service Pack 3 Setup - Feature selection

2.Select authentication mode for the instance you are going to update (Picture 2)

(Picture 2) - Microsoft SQL Server 2005 Service Pack 3 Setup - Authentication

3. Enter credentials of the login you want to run service pack on behalf of.

(Picture 3) - Microsoft SQL Server 2005 Service Pack 3 Setup - Remote User Account

4. Next window shows the processes that lock files to be updated. (Picture 4). You need to stop the processes that prevent sql server update.

(Picture 4) - Microsoft SQL Server 2005 Service Pack 3 Setup - Running processes

Click next.

(Picture 5) - Microsoft SQL Server 2005 Service Pack 3 Setup - Ready to install

5. Look at the updating process and check the result of operations (Picture 6)

(Picture 6) - Microsoft SQL Server 2005 Service Pack 3 Setup - Installation process
6. Summary result of updating SQL tools. Shows all results of updating process. Failed updates can be identified (Picture 7). It is also possible to view the log file clicking “View Summary”. Check and analyse Summary result and the log file.

(Picture 7)- Microsoft SQL Server 2005 Service Pack 3 Setup - Additional information
7. Final step.
After update of the active node you needs to update all inactive nodes. (Tools and non-clustered services i.e. SSRS - sql server reporting services need to be updated on the actual node)

To update the inactive node it is necessary to execute all the steps except steps 2 and 3. There can be some problems with updating database service in the cluster. Restart of each node of the cluster and restart of the cluster service usually resolves all the problems. After restart try to update database service one more time.