title: how to adjust space between buttons equally using AutoLayout
body: <p>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?</p>
  
  <p>the green area keeps its size.</p>
  
  <p><img src="https://i.stack.imgur.com/ZcssV.png" alt="enter image description here">
  <img src="https://i.stack.imgur.com/Mc90M.png" alt="enter image description here"></p>