Flex short hand property fallback

Question

So I'm having an issue with flexbox in css. Particularly when using the flex shorthand property. For example:

If I have flex: 0 0 30%

In IE it just seems to get ignored in certain places. I'm using modernizr so I'm targeting the selector as:

.no-flex{
 .st-col-thirds {
   width: 33%;
  }
}

Which I thought would work... But it doesn't.

Then I tried:

 .no-flex{
  .st-col-thirds {
    min-width: 550px;
    margin: 5px;
   }
 }

This works but it's super hacked and it's not responsive.

This is not a duplicate question.