The problem is I can't make a 3d cut and I don't know where the problem is. It either cuts the entire part or doesn't want to cut at all. I attached the photos so you could see what's wrong and what I am looking for. I want to make the entire body have the same thickness and I don't how to get rid of the inside "useless" thickness.
Are you wanting the inner parts shelled?? I ended up getting your sketch to extrude cut using a surface reference. Not sure its what you want, but it removed the inner parts and left the outside face(the curve).
Answered with a tutorial: https://grabcad.com/tutorials/how-to-cut-extrude-a-3d-sketch-in-solidworks--2
I think you understood correctly. In other words, what I need is the upper surface reflected exactly inside. So the thickness is everywhere 3 mm. Please upload what you've done as a SLDPRT file so i could see whether you understood what I need. Thank you.