the new civic is not intended for three persons on the back, its an executive car not a farmer's car ( like corolla ) where you can stuff as much people as wanted in the car. if you want to sit in the civic then sit behind the front seats, not in the middle of the rear seat, they made a hump there to tell the people like you to not to sit there.
so its not a defect or bad design. that hump is there for a reason..