Not as much protection as you'd think.

Try this at home - get a shovel, and try to pile "loose earth" into the shape you've described (3ft tall, 2ft deep wall). It will immediately fall down into a pile. The angle of repose of soil is 30-45 degrees. In your configuration (2ft diameter base) the pile needs to have an angle of repose of 75 degrees. So the entire thing flops over to about half its height. The soil from the sides of the hole is also falling inwards, filling up your hole, probably reducing the amount of cover from the hole by half as well. If you're unlucky, the soil from the wall will fall backwards into your hole and reduce the depth even further. It would look more like this:

|      OO

|   OOOOOOO

XXXXXXXXXXXO      XXXXXXXXXX

XXXXXXXXXXXOOO XXXXXXXXXXXXX

XXXXXXXXXXXXOXXXXXXXXXXXXXXX

If you're lucky, you'll get 3 feet of cover (partial cover) out of this. There's a reason soldiers build trenches out of packed, not loose, soil.