Short answer: the respect in which God is one is not the respect in which He is three. He is one in being (contra polytheism), but three in hypostasis (contra Sabellianism).
-A8
Nice theological answer, but not what I'm looking for. I'm looking at A8 and asking A8 to explain the mechanics of how God can be three and yet is one. Don't use the big theological terms. Just explain it in "plain English."