how to dig 3 inches deep perfectly level area?

I am making a putting green in my backyard and need to dig 3 inches deep. I did not realize there were such things as sod cutters so i just finished digging out the grass area. Now after raking it, it still is very uneven and not 3 inches deep everywhere. The area is 20×20. The next step is compacting sub base. So do i even need the dirt to be level if i am adding tons of sub base and compacting it?

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