btw, you say you're building a raised flower planter? Do you mean a raised flower bed? You estimated 3 cubic yards of dirt... Assuming you're intelligent enough to make a decent estimate, you don't want to purchase it by the bag. I think a cubic foot of topsoil is about 80 pounds; therefore, those 40 pound bags are 1/2 cubic foot. You're going to need 162 of those 40 pound bags in order to reach your 3 cubic yards. (which makes me remember, it's about 1 1/4 tons per cubic yard... duhhh!) Save yourself a lot of trouble, call a landscaper and have him deliver 3 yards of topsoil. The cost varies, but should run about $100-150ish.
(edit: topsoil, concrete, etc. are sold "by the yard" which is understood to mean "cubic yard")