It would obviously depend on what you want to learn. Do you want to just learn general tips and tricks with just using CC3, or are you looking for specific workflows to be able to do something specific like clothing creation or exporting to a game engine? Or DAZ or Blender workflows to import into CC3?https://forum.reallusion.com/Character-Creator/Training-Tutorials-Hints-and-Tips
along with Reallusions youtube channel and their website https://www.reallusion.com/character-creator/tutorial.html
along with many free webinars they run. There is also plenty of community users who have been creating tutorials for quite a while now I believe. A quick search in youtube "Character Creator Tutorial" will bring up hundreds of tutorials.
Reallusion also has a blog https://blog.reallusion.com/category/artist-tutorials/
I've started a tutorial series for clothing creation using ZBrush, substance painter, and CC3 (also using 3dcoat for some editing and UV's which can still be done in ZBrush) I'm nowhere near an expert in any way. Basically just outlying my workflow. I hope they may help some people, or they may not like the way I produce content and stick to their own workflow.
Also, there are features in CC3 that were present in CC1 and 2 (which those tutorials are in the Iclone 6 tutorials on the youtube channel, but aren't in the CC3 tutorials because they were already covered previously)
Those tutorials are still somewhat relevant for CC3
Also, the best learning resource for anything is.. Google "how do i, etc etc"
(That's how i learn anything i need to know)