Boxing stance tends to have lead leg forward and rear leg behind with a narrow stance shoulder width or less. Your lead shoulder and rear shoulder should be inline that is your medial deltoid of the lead shoulder is facing your opponent. This makes sprawling harder.
A kick boxing stance would have legs farther apart, more than shoulder width and your more squared off to your opponent. Your lead shoulder and trailing shoulders should form a 45degree angle to your opponent whereas the boxing stance is is perpendicular.
The mma stance varies but can nearly parallel to you oponent, both shoulders facing your opponent.
The easiest way to see the difference is to first stand in boxing stance, perpendicular to opponent then move your trailing leg out until it becomes parallel with your right shoulder. I'll post pics later if I can get around to it.