[css] CSS flex, how to display one item on first line and two on the next line

This is a pretty simple question, I guess, but I can't get 3 items in the flex container to display in 2 rows, one in the first row and the other 2 in the second row. This is the CSS for the flex container. It displays the 3 items on a single line, obviously :)

div.intro_container {
  width: 100%;
  display: flex;
  flex-direction: row;
  flex-wrap: nowrap;
}

If flex-wrap is set to wrap, then the 3 items are displayed in a column. I thought the wrap setting was needed to display container items on several lines. I've tried this CSS for the first container item, intending to have it occupy the whole of the first row, but it didn't work

div.intro_item_1 {
  flex-grow: 3;
}

I've followed the instructions in "CSS-Tricks" but I'm really not sure which combination of commands to use. Any help would be very welcome as I'm puzzled by this.

The answer given by Nico O is correct. However this doesn't get the desired result on Internet Explorer 10 to 11 and Firefox.

For IE, I found that changing

.flex > div
{
   flex: 1 0 50%;
}

to

.flex > div
{
   flex: 1 0 45%;
}

seems to do the trick. Don't ask me why, I haven't gone any further into this but it might have something to do with how IE renders the border-box or something.

In the case of Firefox I solved it by adding

display: inline-block;

to the items.