What can I do about water on my garage floor?

In most states I would think 3 years is past the time a contractor legally would have to come back and make things right but you will not know until you ask. As far as keeping out the water, it sounds like there is more than what drips off your car when you drive in.  Check the grade of the driveway and make sure water is running away from the entrance.  You may also need to have someone cut some new drains for your floor to help keep if clean and dry.  Sounds like a pretty major problem if the wallboard is in contact with water…perfect recipe for mold.  Make sure you have this checked out.