I’ve got mixed feelings about that scene. It doesn’t really add brand new information regarding Luke’s motivations, it just lays them out a bit more obviously. It’s also yet another scene of subversion and comic relief in an already long movie that’s already packed with both.
More than either of those, though, it’s a bit odd tonally to imply that not only was Luke not alone in his exile (a big reason Hal removed the caretakers in the first place), but also that they had a monthly party while he was there. It only makes it look more like Luke is running away from his responsibilities for purely selfish reasons, when really his self-exile is supposed to be a form of tragic sacrifice (he’s determined that the Jedi only make things worse, so he’s carrying the burden of the religion to a lonely grave).
Also, just to nitpick, this is technically a continuation of the second lesson scene, not a third lesson. The third lesson never officially occurs, which I think is a deliberate decision on Rian Johnson’s part to leave things open to interpretation. Perhaps the third lesson is the one Rey has to learn for herself when she leaves, or the third lesson is one that Luke himself has to learn from Yoda, or maybe it’s when Luke returns to the light and teaches Rey and Kylo something with his sacrifice. Heck, with TROS, maybe the third lesson is literally when Rey returns to Ach-To and Luke gets to teach her one last time.
Anyway, if Hal decides to add that scene back in I won’t mind terribly, but I don’t think Rian was wrong for cutting it. Much like the alternate Phasma death scene, it’s arguable that it distracts more than it really adds.