In my experience this is usually a network problem. You need a strong, fast, reliable connection for gigabit-sized downloads like this.
I don't usually update via the app store. Instead, I downloading the .dmg file directly from apple support. http://support.apple.com/downloads/DL1905/en_US/macosupdcombo10.12.3.dmg
I experienced the same thing on my macbook pro. Just reboot and keep on retrying. It's usually a connection error.
Hi everyone, thanks for your replies. It is a connection problem. I was finally able to download the files. Thank you!