What could be causing Windows Update to use 3TB of data?

That's a hefty amount, but remember that it might also include downloads from the Microsoft Store or the Xbox app. If you've grabbed some games, that can really add up. Plus, Windows Update has this feature where it shares updates with other devices on your network, and if that's enabled, it could be using a lot of bandwidth that way. You might want to check your delivery optimization settings to see if anything unusual is going on.

Have you configured your Windows Optimization settings to use the internet? If so, your PC might be inadvertently helping to update your neighbors' machines too, which could lead to that massive data consumption.

Are you running multiple virtual machines? Windows Update has a delivery optimization feature that operates like torrenting, sharing updates with local devices. It could be that one VM is downloading updates and then passing them to the others, inflating your data usage.

I've run into a similar situation. If you have several Windows machines on the same network, they can do a peer-to-peer sharing for updates. So, if one device downloads an update, it can share it with the others, which might explain the data spike you're seeing.