What do you think is the purpose of the three-act structure?
I think it acts like a very high-level outline. By separating into three distinct acts, authors/scriptwriters/etc. can then plan out when to describe the setting and characters, when to bring up the conflict and when to get to the Resolution and wrap things up.
Why do you think this structure is the most popular amongst storytellers across the world?
Like @ivanskilling mentioned, it's easy to remember and relatively easy to plan out. Once you have a high-level plan written out in the form of the Three-Act structure, you can then break those down into different scenes.
What issues do you think sticking rigidly to a three-act structure could cause?
At times, I find it a little too rigid. The Three Act structure doesn't seem to allow for subplots, multiple mysteries, a tie of different conflicts into one solution and/or ending, etc. I think that a lot can be lost by sticking rigidly to the structure and some interesting stories can have information missing. Also, is it really just three acts? Couldn't there be multiple arcs with their own beginning, middle, and end?
What novels can you think of that use a simple three act structure?
Hamlet, Romeo and Juliet, Antigone, Medea, a lot of plays have these structures. I think mystery novels and some horror stories (Carrie, IT, Misery, Colors out of Space) do very well with the Three-Act structure too.
Romances commonly use the same structure and often end the same way: "Happily Ever After"...ugh. Kinda boring, I also want to know MORE about the relationship once it's finally settled.
What novels can you think of that break the rules of the three-act structure?
Though it asks Novels, I'm gonna also bring in movies well. Stream of Consciousness novels and Autobiographies ('Catcher in the Rye', 'Trainspotting', etc) often don't follow the structure because they are focused on an actual day in the life, or year in the life of someone.
We then have Irreversible and Memento, which are movies that explicitly break the rules and start from the END (aka Resolution) then go through events backward (or in Memento, forwards AND backward) to reveal precisely how the main character got to that point.
We also have light-novels and serialized stories that have an overarching goal, but the characters go through multiple arcs towards that goal. Often times that goal ends of changing as a result of the resolution of one arc, leading to another conflict in the next (Attack on Titan, Claymore, One Piece, Sword Art Online, insert some anime here...)
Finally, we have novels, movies, and shows which show different events in different time periods finally coalesce into one story. This is done to show the backstory or different journeys of multiple protagonist before we get to the main conflict. An interesting way to do this would be to show the POV of the protag, but also then show the POV of the antag, therefor offering different views to the conflict. I don't think a lot of stories do this, and I really wish they did.