This totally saved me! Thank you!

Well, I think it works: https://github.com/soupday/cc3_unity_scripts/blob/main/Editor/TwistRigCorrection.cs
(Requires the Animation Rigging package.)

Put it in an folder called 'Editor'. It adds two menus CC3->Rigging->Add Twist Rig and Remove Twist Rig. (Use with the character selected in the scene.)




It can be quite a subtle effect, it's difficult to tell if I've done it right.
Also note, I could not get more than one axis to operate on a single twist bone, so I just went with the X axis for the knee and elbow share bones.
It probably also won't work on older pre CC3 character generations as they have some different bone names.

Interesting, I had no idea that this package even existed.

This could possibly be automated, CC3 character's bones always have the same names and structure, so it shoudn't be too hard to put together am editor script that adds all the twist corrections to any CC3 character...

Beginner tutorial for setting up CC3 twist bones and elbows with Unity's Animation Rigging.