CNC-cutting the sides to shape before bending worked surprisingly well. I had left 10mm extra at each end, but that turned out to be unnecessary. The back will have a 3000mm radius and the top will be bent slightly inwards for stability.
He means the amount the back bulges out instead of being a flat plane. The arc of it is curved the same amount as a segment of a circle with a 3 meter radius. The bigger the radius, the less the back curves.
Think a situation like this, except with a smaller surface area:
