The simplest flexbox grid

grid.css

.grid {
  margin: 0 auto;
  max-width: 75rem;
}

.padded {
  padding: 0 2rem;
}

.gutters > .row {
  margin-left: -1rem;
}

.gutters > .row > .column {
  padding: 1rem 0 0 1rem;
}

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

.column {
  flex-grow: 1;
}

grid.html

<div class="grid padded gutters">
  <div class="row">
    <div class="column"></div>
    <div class="column"></div>
    <div class="column"></div>
  </div>
  <div class="row">
    <div class="column"></div>
    <div class="column"></div>
  </div>
  <div class="row">
    <div class="column"></div>
  </div>
</div>

Set a max-width on the .grid element to size it.
Remove the gutters or the padding depending on what you need.