Hello,

Is it possible to change the number of "lines" that are scrolled when I hit the next button ?

When I'm reading a manga in landscape mode, pages are cutted in half, and I would like that they are cutted in 3 parts instead.

I'm not sure to be clear...
No, I don't think it's possible. You could try fiddling with different zoom modes (Top menu -> Second tab -> Switch zoom mode). Alternatively you could try to use Panel zoom, how well it works depends on manga, but it's worth giving a shot if there are clear boundaries between panels.
You can change DOVERLAPPIXELS.

https://github.com/koreader/koreader/blob/25cfef3f6dc5bb690ae64d1648d3291164cc04e5/defaults.lua#L62-L63
thank you ! that's exactly what I was looking for.
Now, I use DOVERLAPPIXELS = 200 instead of DOVERLAPPIXELS = 30.