How do you locate the main drain in a home that is 3 years old with 3 black pipes sticking out of the corner of the floor in the basement?

I know this might be cheating, but you can always contact your local water company and tell them that you are planning to do some digging for some project (make up something plausible!) and ask if they will let you know where all your underground pipes are so you won't damage them while working on your project. I think there is a website also (maybe beforeyoudig.com or something like that). Anyway, they'll come out within a couple of days and mark everything (gas, water, etc) that is underground. They all use different colors, so find out from the water company what colors they'll use to mark which lines. In my area, blue is water into the house and green is waste water out of the house. Also, while this is not 100% guaranteed, locate the vent pipe on the roof of the house, and most times that vent pipe will be directly in line with the sewer drain line.