When contractors say yards they mean cubic yards and the volume of a rectangular slab is the length times the width times the thickness. Since you want the volume in cubic yards you need first to convert all the measurements to yards. There are 3 feet in a yard so
32 feet = 32/3 yards
6 feet = 6/3 = 2 yards
There are 36 inches in a yard so
5 inches = 5/36 yards
Thus the volume of concrete you need is
32/3 2 5/36 = 2.96 cubic yards
Hence you need 3 yards of concrete.