Use FlexBox Justify content property to align elements having two divs and no third

Question

I am trying to use the flexbox .

I have following structure,

    <div className={css.firstContainer}>
         <div className={css.firstChild}>
          content 1
         </div>
         <div className={css.secondChild}>
        content 2
         </div>
     </div>

Now I am trying to divide it in three parts ( does not have third div) so that third row space will be empty.

    . firstContainer {
     display:flex,
     justify-content: space-between
    }

with this first div appears at start and second at end and middle space remains so, I am trying that first div at start then in middle second div and for third space remains empty

How do I achieve this ?