I usually start with a quick search on Expedia or Travelocity just to get a sense for what the standard rates are for the type of car I want. You should have no problem with a basic mid-size car for three people plus luggage, unless you'll have a lot of luggage for some reason.
Most of the rental company Web sites have some kind of indication for how many suitcases and passengers a given car will hold. Be forewarned, however, that when you reserve the reservation likely will provide an example of the type of car you might get, but you'll never know for sure until you get there.
Once you know the ranges, then start trying the codes to see which one will give you the best deal. It's weird, but sometimes a code will give you a bigger discount that you expect and sometimes nothing is available for one code, but things are available for another. There's no magic and it doesn't take an expert; it just takes plain old patience. I've usually gotten the best results with the "one car class upgrade" offers, but that's not always the case.