The movie bombed because it was adapting part of the story that almost no one cared about, filled with character no one knew and they happen to all be orc, so not a lot of people were interested in the first place.
And your source for this is...?
I don't really buy this argument - there's countless film adaptations of obscure works that have been so successful that they've overshadowed the works they were based on (Space Oddyssey, Godfather, Die Hard, Rambo, etc.) And while the argument that humans and orcs may seem limiting, it's not too far removed from something like Lord of the Rings conceptually. Having stuff like night elves thrown in doesn't automatically make the work better.
I can speculate as to why the movie didn't do well, among which being the fact that it's average at best, but this is a film for a general audience. Your average Warcraft fan may be more familiar with WC3/WOW than WC1/2, but to the average filmgoer, that's a moot distinction.
Nobody read the manual of WC3 before starting it,
I did.
Okay, technically I played WC3 in an Internet cafe where I played some of the campaign and had no idea what was happening. After getting the game myself and the manual, things made a lot more sense.
the 1/2 story is almost completely irrelevant to the event of 3.
Not really. The status quo of 3 can't exist without 1 & 2 happening. It's certainly possible to jump into 3 without playing the earlier games (I'd actually reccomend that for newbies), but in terms of lore? You can't ignore the events of the First and Second War when discussing the Third. Without 1, you have no orcs on Azeroth, without 2, you have no Scourge, etc.
All you need to cover is that the demon send the orc over, but that's only relevant for Grom part of the story and can be covered in movie 2 (in like a 5 min scenes) so you don't need to start with exposition for the first movie.
You kind of need to cover a lot more than that. I mean, hypothetically, if I had to start with WC3, then I'd look at how Lord of the Rings handled things. In doing so, I'd have to cover:
-Emergence of the orcs, and the destruction of Stormwind (First War)
-The Second War
-The internment of the orcs, their rebellion, and the Alliance splintering
I can get away without referencing the events of Beyond the Dark Portal, but if you're jumping into the story this late, you need the context behind it. If your story is starting with a fractured Alliance, with orcs revolting against internment camps, and so on, you need to know how things got to that point. Maybe not immediately, but at some point in the story.
WC3 story can be easily adapted to a trilogy, I don't know where your getting 5 movie from, but honestly there's barely enough for 3.
Five movies for five campaigns. And yes, every one of that can cover a movie. You can cut out missions 3-5 of the Horde prequel campaign easily enough (certainly that's what the original release did), but apart from that?
The orc campaign doesn't have that much happening, it can be almost entirely covered in movie 2 (just save Grom/Mannaroth for the start of movie 3) along the undead campaign and starting to setup the NE. Movie 3 is Tyrande/Illidan, finishing with Archemon fight (honestly movie 3 is kinda empty, might need to save more of the orc campaign by covering more of the fluff stuff in movie 2, maybe ending with Grom killing the centaur dude).
You're coming up against the same problems I did. It's hard to mix and match these things, from campaigns that have a beginning, middle, and end, and shift protagonists.
Frankly, if you had to adapt WC3, you could just make it four films. Even if I agreed that little happened in the Horde campaign, you can fill stuff in by including stuff from Jaina's POV, since her forces are active in Kalimdor in the same period Thrall's are. But that's of course me agreeing that it's preferable to jump straight into WC3 from the start. The problem with adapting WC3 (for me) is that you're already adapting a strong story into a different medium. There's less 'return on investment' (storywise) from adapting WC3 than adapting the games prior to it. Whatever the flaws of the actual Warcraft film, it had far more character(s) than the game did for instance.