Hello! in order to have the same part in 2 skins but not the third, place it in a skin placeholder for one of the skins it needs to appear, then make a copy of it, switch to the other skin that needs to share that part too, then place it inside that same skin placeholder, that should be empty now.
As long as you have it in both skins, but empty in the third, everything should look like you want it (:

Didjynn a écrit

thank you, I wanted to avoid to duplicate but it does the job (quite boring when you have as much parts as I do).

If you have many skins then you might want to try this method instead:
Skins - Spine User Guide: New skin placeholder
You remove the skin placeholder, then select the attachment in question, and create a new one. If you're on the latest version of Spine, it will ask you if you want to duplicate the attachment for each skin. You select it, then delete the copy from the skins that don't need it by quickly changing skin pressing , or ..

Didjynn a écrit

Will Spine add the possibility to create folders to organise the bones ?

Just create a bone to put all the bones within. I usually just do that And a folder would basically do the same thing.
I also recommend color coding the bones, and naming them so that the alphabetical sorting groups them together already.