Some user are seeing issues getting connected to the bootload with the duet3, with other cables or computers sometimes being the solution.
But yes, once you're on 3.0 you should be able to update to 3.1.1.
simplet is probably to run M997 in DWC (or via usb serial if DSF/DWC doesn't work yet)
(edit: or with bossa the interim step of 3.0 doesn't apply, its a limitation of the iap firmware which is bypassed with bossa)