Phone round your local DIY sheds and timber merchants, and ask if they can can laminated chipboard to size.
The material is very common https://www.wickes.co.uk/MFC-White-Furniture-Panel---15-x-500-x-2400mm/p/236234 and not expensive
And some outlets have a special very large saw mounted on a flat surface that can cut rectangles to any size you want.
Look at the size of the board you will need, and, as well as your three shelves, consider what other pieces will be useful (you will have to pay for the whole board, so use it all.)
When you go in, with your measurements written on a piece of paper, go first to the cutting desk, to make sure it is staffed and working. You take a board off the rack and wheel it to the cutting desk.
The reason for the piece of paper is that they take it off you before cutting, and keep it in case you come back complaining they don't fit.
The special saw cuts a much straighter and level edge than you could cut yourself.