I was reading the up until the middle of the religion arc thinking that a tv show would be the best way to adapt, but ended up changing my mind. My guess would be for the first film to more or less adapt the Black Swordsman arc, acting as an introduction to the characters and mythos in a relativelly cheap way, trying to make a more simple film with a begining, middle and end.
Then after that they would do Golden Age, you could make an epic 3 hours film if some things in the middle were changed but were able to convey the same idea without rushing the story (the films are an example of what happens when you rush it). Or, you could do a 2 parter epic, each film having around 2 hours and a half, three films seems like king of a stretch in my opinion, as long as each had a decent legth, 2 films should be enough.
Conviction and Millenium Empire would be easier to adapt in a single movie, there's much stuff you can cut or that you need to change in order to work better in live-action, both arcs even have a similar structure in that it starts with a simple story (Fairies/Trolls) and then goes into a larger event.
Considering the pacing and the scale, i just think that cinema would be a better way to adapt this manga. I think it's strange that studios aren't jumping at the oportunity to do this, but then again, Hollywood seems to mostly ignore mangas and animes, even if some of them are perfect for the big screen.