If you mean 3 ft * 3 ft * 1.5 ft = 13.5 ft^3, that is an infinite number of square feet because ft^3 is in 3 dimensions and ft^2 is in 2 dimensions (volume vs area).
If you mean how many cubic feet of dirt do you need for the garden, that should be 13.5 ft^3, 13.5/27 = 0.5 yd^3