Take it to an audio specialty shop. If you have to ask this question, I don't think you have the expertise to build it yourself.
Building up the floor to accomodate a pair of subwoofers means you are a) giving up your spare tire or b) raising the floor at least 8-10" and making it difficult to access your spare. I don't recommend driving around without a spare tire.
Again, take it to an audio specialty shop. They will walk you through the options you have and show you pictures of similar installs they have completed. They might not be in a Mazda3 but they will be of similar hatchback vehicles. If they don't have pictures to show you...go somewhere else.