How to install Service Pack 3 for SharePoint 2007 Server?
1. Download and Install Service Pack 3 for Windows SharePoint Services 2007 (KB2526305), 64-Bit Edition
http://www.microsoft.com/download/en/confirmation.aspx?id=27828
If you have multiple front end SharePoint servers, you need run Service Pack 3 in all servers
2. Download SharePoint 2007 SP3 for 64bit and install in your SharePoint 2007 server
http://www.microsoft.com/download/en/details.aspx?id=27803
Once the update is installed, you will get SharePoint product and technologies configuration wizard. Run the configuration
Wizard to install SP3 udpates
You must run setup to install new binary files for every server in your server farm. If you have
multiple servers in your server farm, run setup and the configuration wizard on the other servers now,
and then return to this server and click OK to continue.
4. Restart your server
Aryan Nava 
Possibly a stupid question:
Would it be wise to take a back-up with a back-up solution or will a normal back-up of servers suffice?
Hi Aryan, I am about to install SP3 on my MOSS 2007 environment shortly. Its currently running at SP1. On some other website I saw that the procedure to apply SP2 is to detach the content databases on all my web applications before applying SP2. Here is that website: http://blogs.architectingconnectedsystems.com/blogs/cjg/archive/2009/05/10/How-to-upgrade-to-SharePoint-2007-SP2-_2D00_-Step-by-Step.aspx
My question is, when applying SP3, would I need to detach content databases or can I just run the WSS and MOSS SP3 packages directly on each on my WFE servers with the content DBs attached?