For the education of those who don't know the expression, "the old three-cornered box" is a hod, a sort of platform on a rod or pole vertically down from its center-bottom, which the bricklayers' helpers would use to carry mortar or bricks up the ladder to the workmen who needed them.

For those who know the song sometimes called "Why Paddy's Not at Work Today", you'll remember that "the gaffer" (the foreman) told Paddy to carry the excess bricks down the ladder in his hod, but "to carry all those bricks by hand, it seemed so very slow", so Paddy worked out the ingenious rope, pulley, and barrel system, which then gave him all the hilarious accidental beating in the song, which explained why he wasn't at work next day.

Should have used "the old three-cornered box"!

Dave Oesterreich