me assume yu have a shape on the ground & then make it 3 inch hi...3 dimenshuns...volume
& yu want volume=1 kubik yard
Volume=area*hite=area*3 inch...=area*0.08333333333333 yard hi
volume=1, so area=volume/hite=1/0.0833333333333333333...
area=12 SQUARE yard (108 square ft)