I would like to build three boxes for my website like the following :

My wish is that the height of boxes 1 & 3 would be equal to the size of box 2, and that this height is  expressed in percentage relatively to the parent element.
What I've tried so far : 
- Box 1 & 3 with display: inline-block; property
- Box 1 & 2 & 3 with fixed height attributes
But I've never managed to have the proper scheme. Can someone explain me how to achieve this ? 
 
    