Download the Exchange 2010 SP3 setup 

http://www.microsoft.com/en-us/download/details.aspx?id=36768

Navigate to the setup file and open setup and click on setup icon which opens the below screen.

Now click on the Install Microsoft Exchange server upgrade.

Now the setup starts copying the files as shown below

Now it takes us to the introduction screen where we just need to click on next

Set up initializes as shown below

Accept the license agreement

During the readiness check you might run into this error as shown below  if you are upgrading from Exchange 2010 SP2 earlier versions to Exchange 2010 SP3.

Note: If you are upgrading from Exchange 2010 Sp2 and later rollups version  to Exchange 2010 Sp3 you will not come across the below error and you can proceed with the upgrade.

Reason why we require this feature to be installed.

Because this feature IIS 6 WMI compatibility is required for the new feature outlook web app mini introduced from Exchange 2010 SP2

What is outlook web app mini

The mini version of Outlook Web App is a lightweight browser-based client, similar to the Outlook Mobile Access client in Exchange 2003. It provides access from simple HTML-compatible browsers that support cookies and it’s designed to be used on a mobile operating system

So we just need to install IIS 6 WMI compatibility on the all CAS servers to get rid of this error

Install the IIS6 WMI compatibility.

Rerun the setup and  readiness check will be successful.

Now click on upgrade and the setup will be successful.

Once upgrade is completed you just need to click on Finish and reboot the server which completes the SP3 installation.