If you need two or three masters on the weight axis depends on the design of the font. So you can try to us only two, interpolate the instances and see if you like the result. If not, you add add the Regular in the middle. If you need corrections only on some glyphs, you can add add extra 'masters' per glyph (in Glyphs.app with a brace layer and with an extra UFO in designspace files, I believe). That way you save a lot work.