>>712716365In Fellowship the extended scenes mostly serve their function well, they expand the movie with worldbuilding and additional context for characters and their motivations.
In Towers some of them are alright but the forced comedy starts becoming noticeable and some of the scenes are somewhat ill-fitting. They interrupt the flow. But it's not too bad, it still works and you won't find any fault in it if you're a fan of LOTR.
In Return however many of the extended scenes are comically shit. Most of it is just humor that even capeshit movies would sneer at for being too juvenile, it's stupid shit that ruins the pacing like skull avalanches or Saruman and Theoden exchanging crass insults (which culminates in that bizarrely brutal slam fall of Saruman on that wheel that plays out like a comedy scene), or the dead cheekily trolling Aragorn for no real reason, or Sam and Frodo fucking around with Orcs from southeast London, or Gimli burping.
Return is best watched with the theatrical edition because most of the extended scenes suck and it really stretches the runtime into absurdity.