It should be possible to select an arbitrary set of edges in 3D, and then click "chamfer/fillet", and have it nicely modified. Sometimes you can do this in a sketch with arcs, but often it is very tedious or impossible.

Most professional CAD tools include these as basic operations. For some reason they are usually separate tools but I think it makes more sense to have it as one tool with a "chamfer style" option which is "straight" or "rounded" (and a radius option).