how to adjust space between buttons equally using AutoLayout

I have a main screen with three button vertically aligned. I want adopt my app for 4" screen. so I want to adjust space between buttons equally to fill the screen. see the below images: how can I accomplish that using AutoLayout?

the green area keeps its size.

enter image description here

Solution

Edit: Apple now favors the use of stack views for this purpose. See this WWDC 2013 video for OS X and this iOS one from WWDC 2015

~~The Apple approved way of doing this (look for section titled "Creating Equal Spacing Between Views")~~ is to insert hidden views between each of your buttons and set those views' heights to be equal.

See my answer over here for some discussion on why this approach makes sense.