Since you want the volume in cubic yards I would first convert all the measurements to yards. There are 3 feet in a yard and hence 32 = 9 square feet in a square yard. Thus
750 square feet is 750/9 = 83.33 square yards.
There are 36 inches in a yard so
3 inches is 3/36 = 0.0833 yards.
Thus the volume of soil needed is
83.33 0.0833 = 6.9 cubic yards.