If like me you have Auto Updates enabled for you Exchange server ( just notify!) then you may have noticed that Exchange 2010 sp3 roll up 3 has been released. If you try and install it automatically then you may see the following error. Additionally https://twitter.com/RhoderickMilne has notified me that this will also error with manual installation of Sp3 update 2.
The error is what it is, the roll up cannot find your exchangeserver.msi, chances are you removed it after you last update to SP3. To fix this issue we need to download Exchange 2010 Sp3 again from here.
Once done you need to extract the contents of the EXE to a folder some where on your server. You can do that by right clicking and selecting extract or simply run the service pack as an Administrator and it will prompt you where to extract the contents to.
Once extracted run the Auto-update process again, now when you are prompted to browse to the location of the exchangeserver.msi then point it to the folder you extracted. The Exchange 2010 Roll up 2 SP3 update will now install without any issues.