If it's like the U-shaped in our Flagstaff, then what you do is move the back sit-on cushion forward to the edge of the table, put the back backrest cushion beside it, and in the hollow at the back you fill in with the two little backrest cushions from the sides. The corner backrest cushions get discarded (i.e. tossed on the sofa).
They never seem to fit right and cushions are constantly shifting, but that's the way we do it. Yours might be this way, or it might not. Don't know. The other option is to just go get an inflatable mattress to use in place of the jigsaw puzzle of back/seat cushions. That's what I keep telling myself I have planned, but so far, we still haven't.
There's no use crying over spilled milk, unless it's on your keyboard.