By default pressing your "X" key will cycle standing to crouch, pressing "X" again will stand back up. "Z" for prone. "Ctrl" for your stance variations between your three stance choices.

LonoB - correct, so how does that help you go to the intermediate crouch?

Your solution does not address my request.
There are 2 stances in between prone and normal crouch.

I want to go from semi crouch to crouch and back again , with one key press and further more to assign the process to the mouse.

Also, you have to stop walking, then press ctrl and W to go up a stance then resume walking, the current stance cycle needs to be taken away from walk.

If you go into the key bindings, there is a section for you to map your own key bindings to custom controls. I assume here you can have your stance adjust and up/down bound to a single key to accomplish what you're wanting to do. Otherwise, I would just say get used to pressing multiple keys.

If applicable, just have stance adjust bound to a mouse key, like Mouse 4 or 5, or higher if you have the available buttons on your mouse.

Covered here in more detail - http://forums.bistudio.com/showthread.php?148836-Suggestions-for-Streamlining-Fixing-Stance-Adjustment-amp-Movement-Speed-Controls/page3

I really don't understand people downvoting this request. If you want to keep using your movement keys (WASD) to adjust your stances that's fine, but why downvote others who'd maybe like to to do things a bit differently? The ARMA ethos has always been about customizability and player choice. On top of everything, this would in no way hinder, handicap or lessen the experiences of the people who like the controls as they are now, so there is absolutely zero need for controversy. I'm extremely tempted to flame some of the blatant ignorance and unwarranted umbrage already on display here, but there's no reason for something that should be so straightforward and self-evident to turn ugly.

I would love to see this change implemented, customization is key- especially in a simulator.

This is covered by #1270.