If your adding to the room to make it wider and not longer your talking ripping off the entire roof off that section and redoing the entire roofline.
Honestly your looking at 50-75k to add a third stall and extended room doing it that way. If you leave the room above and do a single pitch roof off the side it would be much much cheaper.
It appears they used attic trusses to make that room which means that her entire office would be ripped out and thrown away to make the room/roof cover the entire 3 car garage.
Ya they did. Did not even think about that. So ya it would be $$$.