The 3GB download, more or less, sounds like the Windows support files that Bootcamp downloads, but, that's not Windows, just the support drivers that Windows needs for your Mac hardware -- and those drivers don't get installed until the Windows install is complete.
Keep in mind that the Boot Camp Assistant does not install Windows. It
prepares a partition that the Windows installer (which you provide) will accept as a destination.
With that in mind, what version of Windows do you have ready to install - probably on a USB flash drive (?)
If you are trying to monitor that process through your Activity Monitor, that also means that Windows has not yet begun to install, because you are still booted to your Mac system. The last step that the Boot Camp Assistant does, is to restart your Mac, so it will boot to the Windows installer that you have ready. (Be careful that you don't choose your
macOS partition for the Windows install. That is never a "happy" result.
)
