Hi Linda.
Area times depth equals volume, so let's get things in the right units and multiply:
1200 square feet is 1200/(3 3) = 400/3 square yards.
3 inches is 3/(12 3) = 1/12 yards.
400/3 square yards times 1/12 yards is 400/36 cubic yards.
That's 11 and 1/9 cubic yards you'll need to fill that space.
Stephen La Rocque.
