My third seat is anouther bench, or make that a dog bench...They are a replacement for the bench seat and nowhere near as comfortable with less leg room. They are used as the middle seats in the six place option.
The 'jump' seats are much more flexible than the bench. Easy in/out, can be stored in the rear baggage. Very handy especially if you carry if you carry pax one way and cargo the other. You can stow long cargo like an outboard motor on one side and a passenger on the other. You can have the bolts permanently installed and use wing nuts to fasten the seats to the side wall. Quick & clean, easy-peasey.