please advice on how to successfull make a 3mm chamfer on highlighted edges chain.
it's coreblock created with moldtooling
You could extrude a surface up to the the face of the block, and then make a ruled surface from the edge, using the ruled surface to cut the corner off the block.
Or just project a sketch onto the block and do the same thing.
You could sweep an angled line (as a surface) around the block using the square base as a path but using the top edges as the guide curve. This should give you a surface to cut the edges off with.
Offset the top edge 3mm on each side, and extrude that at an angle of 45 deg, and do it on all 4 sides.
Assuming you'd cut the chamfer with a wire edm?