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

You can do something like this:

.flex {
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
}

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

.flex>div:first-child {
  flex: 0 1 100%;
}

<div class="flex">
  <div>Hi</div>
  <div>Hello</div>
  <div>Hello 2</div>
</div>

Here is a demo: http://jsfiddle.net/73574emn/1/

This model relies on the line-wrap after one "row" is full. Since we set the first item's flex-basis to be 100% it fills the first row completely. Special attention on the flex-wrap: wrap;

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.