If you are talking about doing a bumpout cab for a sink or cooktop base, I don't believe the software supports anything that would enable you to clip corners.
I could be wrong, however. I'm not a guru.
May we presume your base is to have a faceframe? You might be able to make a frameless cab, then use the part editor to clip its parts as required, then make three faceframes (faceframed cabs with everything deleted except the faceframes), then make an assembly out of the whole shebang.
But as for doing all this as one seed, using the cabinet editor, forgettaboutit.
Gene is right...you will have to use the Part Editor to clip the corners on the various parts (top, deck, shelves).
Take one of the parts into the Part Editor. Select the Chamfer tool and at the bottom of the screen, select \"1 Distance\". Enter 3 in the \"Enter Chamfer Distance\" box and press enter.
Now, click on the edge labeled \"Front\" then click on one of the sides (top or bottom in the display)--the right side is at the top of the display.
Do this for all parts that need to be clipped.
Save the cabinet and you can reuse it as a seed when you need a clipped corner cabinet again.
You must inset the side of the cabinet and give the face frame a negative scribe value then chamfer the shelves and top and deck as Dan described. Use a display board or panel to complete the corner. You can add flutes etc to this display part.