rectangular prisms

Same way you do when the sides are integers. For example, if the three sides are 3/4, 1 1/2. and 3 1/2, convert the mixed numbers to improper fractions, multiply all numerators, multiply all denominators, and convert your answer to a mixed number (if greater than 1) in lowest terms...

3/4 * 3/2 * 7/2 = (3*3*7) / (4*2*2) = 63/16 = 3 15/16