An abacus mnemonic for adding 3: '(slide) three down, then five-minus-two' achieves +3 on a column. Because 算盘 abacus calculations are famously fast, the chant became shorthand for doing anything 'in three quick moves'.
usage
Adverbial; pairs with 就 + verb. The verb is usually a manual or short action — eating, packing, fixing, finishing.
