[Noumenon]

They're already there with Six Day underwear. Three leg holes allow the wearer to rotate them once for each of three days, and then turn them inside out for another three days of rotation.

There is an ongoing dispute over the best direction for rotation - clockwise vs counter-clockwise.

[wimpycat]

And don't forget inside-out.

[hellinahandcart]

Three leg holes allow the wearer to rotate them once for each of three days, and then turn them inside out for another three days of rotation.

That's gonna leave one ugly panty line.