I'd never heard of this "rule" but it instantly appealed to me. Last year I worked hard on the Wardrobe Architect project, and by choosing a color palette (navy, pink and grey) I've cut down tremendously on random fabric purchases. Y'know, the "oooo pretty" ones that never turn in to anything?
After solidifying a color scheme, I think the next step in a sensible closet is to eliminate orphans. Those sad, lonely pieces without a mate, that are fine alone but don't make an outfit. I have lots of orphans and it makes me sad. This rule of three would certainly take care of that problem.
I love this cardigan, but it doesn't go with anything.
Do you plan entire outfits in one go, or are you like me, typically only thinking ahead one garment at a time?