Hello,
I'm trying to create a 3 colors palette from any image. Basically I would like it to be split into shadows, midtones and highlights.
If you know GIMP, you can do that by using "Index" mode for the image. In Photoshop you can do "export for Web" and it allows you to extract those 3 colors.
Any way to do that in Krita?

If you're interested in shadows, midtones and highlights, then Filter -> Artistic -> Index Colours may be useful for you. It looks like you can assign different colours to different parts of the shadows. midtones and highlights range. (I've only ever 'played' with it.)

The G'MIC filters may have something that does what you need but there are many of them and they often have names that don't make their function obvious.