Responsive site using HTML5 and CSS3

Here could be an interesting way to go about it =>

.container {
  display: flex;
  flex-flow: row wrap;
}

.container > div {
  flex-basis: 370px;
  flex-grow: 1;
}

I tried using grid, but it's impossible to set the column span behavior on wrapped items and needed that 3rd item to span 2 columns when wrapped.