To me the case and 3 bosses look like solid geometry in the picture, so 'surface trim' would not work because that's a tool for surface geometry and features, not solids.
If so, 'Delete face' would be the first feature I would try for eliminating the 3 bosses, 'insert>face>delete' and select all the upright faces and the tops of the 3 bosses (21 total faces should be selected). That should leave you with a clean outer case shell.
It worked, thank you very much.
I think making a sketch on a workplane in space above them that encompasses all the shapes should be done first. Then do an extrude,cut to face option down to the surface they rest on could work.
I solved the problem. But thanks for your help. Next time I will do it your way, more simple.
try the 'intersect' tool
Also, if you are able to upload your model it will give the community the opportunity to poke around and I bet you will a number of different ways to get the results you desire. It is hard to tell exactly what you are working with from a single picture.
I solved the problem. But next time I have problem, I will do that way. Thanks for suggestion.