Take Three From Five

Choose any five integers you like — positive, negative, repeats, whatever you want — and enter them below. Click Submit and the tool will search for three of your five numbers that add up to a multiple of 3. Try to pick a set that doesn't have one — you won't be able to! No matter which five integers you choose, there's always at least one such trio. Once you've tried a few sets, see if you can work out why that's always true.