How flexible the seating is in an S-Class depends largely on what seating package you specify when you order the car. For example, if you choose the Rear Seat Comfort Package that has two reclining rear seats with massage functionality, there will not be space for three people in the back of the car.
However, regardless of the seating layout you order, the rear seats cannot fold down to extend the boot space. This feature isn’t even offered as an option so if you need more flexibility look at a Range Rover instead.
Things are better up front, where even on the entry-level 350d AMG Line, the front passenger seat is electrically powered and has four-way lumbar adjustment.