I like the idea of adding the lesson 3 to TROS, but, in defense of the lack of a third lesson in TLJ…
Rey’s third lesson is something she “learns” on her own. Don’t nail your destiny on anyone, or you’ll be disappointed. Basically.
But more importantly; screw this religion with its specific rules and lessons. She metaphorically doesn’t NEED a third lesson taught to her in the old Jedi way, because its about doing things your own way for the greater good. Not following a specific, flawed guideline. Something like that.
Also Luke didn’t get to teach her the third lesson