be (as) easy as one-two-three

To be extremely easy, simple, or intuitive; to require very little skill or effort. (Sometimes used with the numbers spelled out ("one-two-three") or written numerically ("123" or "1-2-3").) After so many years as an accountant, doing taxes is as easy as one-two-three for me. Boy, that test was easy as 1-2-3! Oh please, I've been playing guitar for 20 years—that song is as easy as 1-2-3.