"hinging on metal pegs going through metal welded to the body"

No damage is necessary to remove the 3rd seat. Take a close look at where those hinges go through the metal welded to the body. One of the "hinge pins" is spring loaded (sort of like a toilet paper holder). Simply push the pin in toward the center of the car and it should come out of the metal bracket on that side of the seat. Lift that side of the seat upward, away from the metal bracket. Then, you can shift the other side of the seat lift the seat away from its metal bracket.

It will all appear logical once the seat is out!

Hope this helps.