Coroplast sheets come in 48" x 96" (4' x 8'). For a 3x4 grid, you'll have enough length for the 4-grid length, but you won't have enough on the 3-grid length. Each grid is 14" plus the space between the grids for the connectors: a 3-grid side needs 41" plus 12" for each side, making it 53" (you'd be 5" short) - unless you make the sides 3 1/2" high instead of 6" if you think that will work.
How about a 2 1/2 grid x 5 grid? That would give you one solid piece that you'll use a piece that's 48" x 83" - less cutting and you get 6" sides. Might be a bit easier to clean than a 3x4 also.