In your original post, you referred to milled carbon fiber epoxy composite so I searched for the linked document just now. I was not familiar with that term when I first answered your question. I was familar with carbon fiber epoxy composite where the carbon fiber is long strands of unidirectional fibers or woven fabrics. Layups of plies of carbon fiber make laminates.
I am familiar with aluminum honeycomb core sandwich structures that have aluminum or carbon fiber laminate face sheets that are bonded to the aluminum honeycomb core using epoxy.
You use the word “infill”, which is associated with an additive design and manufacturing process that results in printing the part in a 3D printer. Infill is the process where a thick section of solid geometry is hollowed out and an infill pattern fills the interior. The infill pattern may be automatically created by the printer or it may be more purposefully designed and be present in the geometry sent to the printer. Is this what you mean when you use the word infill?
If so, I expect you want to print the honeycomb core pattern. The hexagonal shape of the cell is created by the manufacturing process of laying down ribbons of aluminum foil with alternating patterns of epoxy to hold the ribbons together. When the stack of ribbons are stretched out, the alternating pattern of epoxy causes a hexagonal shape. But if you are printing with milled CF epoxy material, there is no need to confine yourself to the hexagonal shape. You could use a square shaped cell instead. The benefit of the square shape is that it can be aligned with the printing direction so the ribs in those directions will be smoother.
I don’t know what honeycomb creator is. I would just use SpaceClaim. Make a copy of the solid body for later. Use the Shell tool to create the outer shell for your part. Make a copy of the shell part for later. Subtract the shell solid from the original solid, now you have the interior solid. Create a pattern for the infill. For example, make a cube, copy that to a 2D grid of cubes, then copy that grid into as many layers as you need to cover the thickness of the interior part. Subtract the 3D grid of cubes from your interior solid, that is your infill. Use the Combine tool to add the infill to the shell solid.
Once you have designed the geometry with the specific infill pattern, you can perform the 3-point bend test.
Once you have the 3-point bending result or more simply to support one end and apply a moment on the other end. The simulation will give you the deflection of the free end where the moment was applied. Use beam deflection equations to solve for Ixx. Apply the moment about the other axis to get the deflection to solve for Iyy.