Split command not working

So I have this odd shell shape and I’m trying to split it (example below) and the command doesn’t seem to be working, maybe I’m doing something wrong. I want to split it along the red line and split that in thirds so the designated blue area will be separate from the above shell and the designated purple area will be taken out.

_Split works with points on curves, isocurves on surfaces (but not on polysurfaces), and curves on curves, surfaces and polysurfaces. But from the picture it looks like you want to use _ExtractSrf (Solid>Extract surface).

I was able to separate the polysurface with extractsrf, but I’m not able to cut it with the red plane that I created.