I want to backup one computer files and two external drives to an 8TB external drive in Mac. How best to do that?
Thank you.
Hi @jsctpc, welcome to the forum 
You can follow the regular setup from here:
For the destination, choose the 8TB drive.
For the sources, you choose the local folders, and the two mounted external drives.
You can set the Volume Size on the last page a bit higher, like 200MiB, to reduce the number of files created, but this is optional.
If you want the backup to also run if one of the external drives are not plugged in, you can set the advanced option --allow-missing-source=true. If you do not set it, Duplicati will give an error when attempting to run the backup.
You can also consider if the data is better suited for 3 individual backups, rather than being stored together. This would later give you an option to move parts of the backup to other storage.