Use an offset x-section instead of planar and create the "cut" with a 120 degree (or whatever shape) slice. (You sketch the section that creates the cross section.) (I'm assuming your part/assembly is cylindrical, but that really doesn't matter.) This also works for the old-fashioned x-sections that jog through holes in different positions on a part. If this is an assembly, you can also section through some components and exclude others. It also helps to think of these in the methods you learned in drafting class instead of which option to pick in the program. (although an "offset" cross section doesn't make much sense).