There's a threaqd for this already but I doon't have time to search for it. Anyhow for a 3 ton box it's pretty simplified. 4x4x8 will fot enough so you only have to cut up a single piece of plywood. I've been meaning to get some pictures up but kep forgetting to take a camera with me to specific houses... Most people will build the one 4x4 wall like a guillotine with removeable slats. Once you remove the coal from the slats you can take them out. Once you get to the bottom you can walk into the bin.
One thing that is important is to have a removeable lid if you put one on. Either on hinges or so it just comes off. This allows for easy delivery, even if your doing it yourself.