What can I do when macOS does not boot an APFS volume anymore and I cannot unlock it?
Regarding the first attempt to simply use unlockVolume, please consider trying this variation instead:

diskutil apfs unlockVolume /dev/disk3s1 -user disk


Your attempt to unlock using the UUID failed, because you failed to enter the complete UUID. I don't know if you typed it in manually or used cut-n-paste, but you've missed the last letter "C" in the UUID - making it one character short. 

I would try that command again with the full UUID - and remember that you're not supposed to enter your passphrase for the drive to unlock it with that UUID - instead you need to enter the personal recovery key. The personal recovery key was presented to you when you first enabled FileVault (I assume this drive was previously a boot drive) - and it takes the form of 24 upper cases alpha-numeric characters, grouped in 4 seperated by dashes. Something like AB1C-DEF2-G3HI-4JKL-MNO5-PQRS.

If the above fails, you can always erase everything on the drive completely, as you indicate this would be an acceptable option.

You can do that by opening Terminal.app and running the following command:

dd if=/dev/zero of=/dev/rdisk3 bs=1m


After it has finished, reboot the system so that it forgets the old partitioning information. Then you can use Disk Utility to add a partition table and format the drive.