Why does time machine backup take "3 days" over USB 3?
This is not normal if every backup takes that long and/or it never finishes.

You should let the backup end if at all possible. Each time you interrupt a backup - you add even more time to check the slow work that was made and then pick up where it left off. If you interrupt a back up too many times, it could take a week to get over a problem spot.

You could:


Let it run and see if the next time is quick.
Interrupt it and delete the partial backup
Clear the backups and start with a clean slate if you don't need the backups or don't care to troubleshoot a temporary issue.


If you want to troubleshoot, you have several options. Apple has a tmdiagnose tool that they use to collect logs and statistics relating to items that can cause problems. 

If you can get help from Apple, I would do that and let them provide the training and expertise needed to make sense of the volumes of data that the diagnosis provides.

There are some other things you might try in this article:


Time Machine: Troubleshooting backup issues


Feel free to post back a more specific question if you find the issue doesn't resolve itself or you need more information about how to interpret your results from the article or the diagnosis tool. Also as you commented, it could simply be a bad hardware situation.