First time posting here so sorry in advance for any mistakes!

I need to split a body in three parts with an offset from the bottom and top surface. The part is NOT a simple boss/base extrude, as it is a section of a wind turbine blade. I need to make an offset from the bottom plane, but cant get it to work with offset surface and the split functions. Which function should I use?

The section of the wind turbine blade needs to be splitted in three parts because the section consist of different materials and we want to test this section in FEM.