We have 3 major stances: stand up, crouch and prone. Each has 3 heights.
Holding down the normal buttons for stand up, crouch and prone (Y,X,C) as a modifier + mousewheel to cycle thru the three levels of their major stance.
Then you can push the major stance button and then fine tune.
For example: If you are running push/hold Y to go prone and then mousewheel to go to the sitting position. Or push X to crouch and then just hold X and mousewheel to adjust to a lower crouch position. You dont have to go thru all the stances every time. You can change to one of the major stances like in Arma2 and fine tune without any other button. And you dont need an additionally modifier button to control the sub stances.