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!
easy as one-two-three
Extremely easy. Come on, give me a hard one! These equations are easy as one-two-three!