Writing flexbox code for 2-column and 3-column on desktop and mobile (wrap)

Here's how you do it for the three columns. I'm only adding that, because it's a bit more tricky:

.container {
  display: flex;
  flex-wrap: wrap;
  flex-direction: row;
  justify-content: flex-start;
  align-items: stretch;
}

.left {
  order: 1;
  background: red;
  flex-basis: 100%;
  height: 300px
}

.middle {
  order: 3;
  background: green;
  flex-basis: 100%;
  height: 300px;
}

.right {
  order: 2;
  background: yellow;
  flex-basis: 100%;
  height: 300px;
}

@media screen and (min-width:600px) {
  .container {
    flex-wrap: nowrap;
  }
  .left {
    flex-basis: 200px;
    order: 1;
  }
  .middle {
    flex-basis: 1;
    order: 2;
  }
  .right {
    flex-basis: 200px;
    order: 3;
  }
}

<div class="container">
  <div class="left"></div>
  <div class="middle"></div>
  <div class="right"></div>
</div>

And the fiddle http://jsfiddle.net/2touox81/

As you can see, you can set column order for flex items.

Hope this helps.

I will assume desktop means screen wider than 600px, mobile less.

The 2-column layout is very simple:

body {
  display: flex;   /* Magic begins */
  flex-wrap: wrap; /* Allow multiple lines */
}
#left, #right {
  flex: 1 300px;   /* Initial width of 600px/2
                      Grow to fill remaining space */
  min-width: 0;    /* No minimal width */
}

body {
  display: flex;
  flex-wrap: wrap;
  text-align: center;
  font-weight: bold;
}
#left, #right {
  flex: 1 300px;
  min-width: 0;
  background: #f55;
  padding: 15px 0;
}
#right {
  background: #57f;
}

<div id="left">Left</div>
<div id="right">Right</div>

The 3-column is only a bit more complex:

body {
  display: flex;            /* Magic begins */
}
#left, #right, #middle {
  min-width: 0;             /* No minimal width */
}
#left, #right {
  flex-basis: 200px;        /* Initial width */
}
#middle {
  flex: 1;                  /* Take up remaining space */
}
@media screen and (max-width: 600px) { /* Mobile */
  body {
    flex-direction: column; /* Column layout */
  }
  #left, #right {
    flex-basis: auto;       /* Unset previous 200px */
  }
  #middle {
    order: 1;               /* Move to the end */
  }
}

body {
  display: flex;
  text-align: center;
  font-weight: bold;
}
#left, #right, #middle {
  min-width: 0;
  padding: 15px 0;
}
#left, #right {
  flex-basis: 200px;
  background: #f55;
}
#right {
  background: #57f;
}
#middle {
  flex: 1;
  background: #5f6;
}
@media screen and (max-width: 600px) {
  body {
    flex-direction: column;
  }
  #left, #right {
    flex-basis: auto;
  }
  #middle {
    order: 1;
  }
}

<div id="left">Left</div>
<div id="middle">Middle</div>
<div id="right">Right</div>