I can’t tell exactly, Xavier, but I would check that you have added the 3 bones inside the same armature object.
Often students accidentally add 3 armatures, instead of 3 bones (inside 1 armature)
If you could post some screen shots or your file, I can look into it further.
You can post it as a submission if you want me to check – or link it here via dropbox or something similar.
