You were running electrical out to a garage...why three feet deep? I dug plenty of trenches this summer, deepest I went was about 10 inches, enough so the bobcat wouldn't break the conduit.
code. I think it was 2 foot minimum, but they request a bit deeper, and hubby did it. He dug it nearly 3, so that the caution tape you run in the trench above it was about 2 feet deep. That way, when someone starts digging, they hit the tape first, and know they're about to strike cable.