If you have a stack of items, you can pick up half of them by right-clicking on it, which is very convenient when you're dividing out a stack on the crafting grid to make many copies of a recipe that needs that item in 2 or 4 slots; but for some recipes, you need to put the same item in 3 or 6 slots, and there's no built-in way to divide a stack into thirds.
So I made an AHK script that, when I held down a key, would
- pick up half of the items in the stack I'm pointing at (right-click) - move the mouse one inventory space to the right - put them down (left-click) - pick up half of them again - move back one inventory space left - put them down, adding them back to the original stack - and repeat
and it would only stop after one of the "pick up" steps; so, after a few iterations, you'd be left with the stack divided into equal thirds, two in your inventory & one on the mouse.
Eg, starting with a stack of 60, it goes 30/30, 45/15, 22/38, 41/19, 20/40, 40/20, 20/40, 40/20... then release the key, and you've got three stacks of 20.