Do I have to disable Avast before downloading sp3? If so, how do I do this?
Disable Avast by right click on the tray icon and then Avast shields control.
But you dont really need to turn Avast of.
Download SP3 and save it to your desktop or other location that you can find later. That way you aren’t unprotected (avast isn’t disabled) whilst downloading the large SP3 update. Now it can be installed whilst your system is off-line, not connected to the internet.
Whilst you shouldn’t need to disable avast, if you have disconnected from the internet then it wouldn’t be as much of a risk if you did temporarily disable avast.
thank you David! 
You’re welcome.