Fellowship's advantage is that it's linear story, the other two are running 3-4 stories throughout. I will say though in the last two films defence that it's probably the best anyone could have made from such a convoluted story as the book was. The book has this really bizarre pacing which wasn't easily adaptable. As for the Hobbit, it's just an excessive amount of film for such a short story, it really feels like Jackson has been trying to recapture what was done 14 years ago, and the results aren't spectacular.
The problem with LOTR is that it isn't one single narrative from Point A to Point B to Point C. This mostly holds true for Fellowship of the Ring, aside from some considerable flashbacks and stuff happening off-screen.
It is actually written as six books, with a prologue and appendix.
Prologue - The creation of the rings, and the journey of the One Ring from Sauron to Isildur to Deagol to Smeagol to Bilbo.
Book 1 - The Ring Sets Out - The discovery of the One Ring and the journey to Rivendell while pursued by the Nine. It ends upon reaching Rivendell
Book 2 - The Ring Goes South - The formation of the Fellowship and journey through Moria and Lothlorien. It ends with the breakup of the Fellowship and Sam and Frodo going to Mordor alone.
Book 3 - The Treason of Isengard - About the Rohan and their war against Saruman. Ends with Saruman's defeat at Orthanc.
Book 4 - The Ring Goes East - Frodo and Sam meet up with Gollum and they journey to Mordor. Ends with Gollum's betrayal, Frodo seemingly being killed (actually captured by orcs), and Sam taking up the Ring and defeating Shelob.
Book 5 - The War of the Ring - Mainly about the Siege of Gondor and Aragorn's return to the throne. Ends with Aragorn leading an army at the Black Gate.
Book 6 - The End of the Third Age - Sam rescues Frodo from Cirith Ungol, The Destruction of the Ring, the journey home, the Scouring of the Shire, and the Ringbearers departing Middle Earth.
Appendix - An epilogue of what happened in the Fourth Age. Ends with Aragorn dying of old age, Arwen pining to death, and Legolas building a boat so he and Gimli can sail to the Undying Lands. Also contains some other stories about the Fall of Numenor and such.
Frodo & Sam don't even appear in Books 3 and 5, and likewise the rest of the Fellowship have nothing to do with Book 4. The timeline is also a bit wonky. For example, the fight against Shelob in Book 4 chronologically occurs at the same time as the Battle of Pelennor Fields two-thirds of the way into Book 5. It was not an easy book to translate to film.
The Hobbit is different. It is only about the same length of any of the six books that make up LOTR, and Jackson has stretched it out to fit three full-length films.
Note that they weren't originally supposed to be released as six books. Tolkien intended them to be released as one, but that is how he structured them.