Why are superhero movies signed by actors in three-parts?

_{Note: it's not only with superhero films but trilogies in general and actor sign contract based on the initial plannings.}

Because of Rule of three

The Latin phrase "omne trium perfectum" (everything that comes in threes is perfect, or, every set of three is complete) conveys the same idea as the rule of three.

Three is magic number to convey a story or even a scene. It's quite common trope in literature too to start with trilogy but lately they get expended later on with prequels and sequels.

And making it in three parts can also act as extended Three-act structure

Take Dark Knight trilogy as an example:

Batman Begins can act as Act 1: Setup; we introduced with main protagonist and see his origin and reasoning for what he do.

The Dark Knight can act as Act 2: Confrontation; here stakes get higher and hero get confrontation with bigger problem. He got retired and kind of broken.

The Dark Knight Rises can act as Act 3: Resolution; we got resolution of the story which started with first film and hero's name get cleared and he died as martyr (or not died).

Spider-Man can't fall into that structure as it was never suppose to stop at three but it can be applied to many 3 part films.

First of all, it's not only the superhero movies that has a trilogy, other movies such as Godfather, Twilight, Lord of the rings etc. also ends up like this.

I am giving my personal opinion here.

I think the concept of trilogy was introduced long before and somehow our mindset has been adapted to it (we just love the number 3). Moreover, producing a sequel of a movie gives the audience a thrilling experience and when you stops at third sequel, we all are (kind of)satisfied. That's how our mind works. Also after a trilogy, the filmmakers can start the story afresh if they want to recreate the character. This is what we can see in Spiderman (starring Maguire, Garfield and Holland) where the storyline changed.

However, some sequels such as James Bond, Transformers keep on coming for the fact that they can make more sequels without any proper introduction of the characters involved in and they know that it's gonna be a box office hit.

Because every story can be split in three parts:

• heroes presentation/plot bonding
• main story
• resolution / explanation

This system is firstly seen in Greek tragedies and upon them almost every story share the same system. Antigone by Sophocles, Little Red Riding Hood, Star Wars.

There are many subparts of each segment (like in the first one you can have hero equipped with magic item or in the last one the hero get the princess) but these are three main parts of EUROPEAN and European based stories (so USA Hollywood). Each subpart is usually also divided into three.

So in SW you have introductions, hero presentation (which also is split in three parts) and embarking on a journey in the first part.

Making three movies is the same system but with added money making idea. Why fit story in one when you can gain money from three of them. Hobbit is the pinnacle of such move. And blatantly obvious with the cutting. First movie is hero presentation and embarking, second is the whole story and third part is resolution.