Use flexboxes! Here is a JSFiddle that I've tested in Chrome.
I specified display: flex
on .a,.b,.c
and specified margin: auto
on .inside
To make this cross browser, just make sure you use all the vendor prefixes.
You can learn more about flexboxes here: http://coding.smashingmagazine.com/2013/05/22/centering-elements-with-flexbox/