“rowspan” behavior with flexbox

Answer 1

is this what you are looking for?

.foo {
  display: flex;
  flex-wrap: wrap;
  flex-direction: column;
  height: 200px;
}

.a, .c {
  flex: 0 0 50%;
  background: green;
}

.b {
  flex: 0 0 100%;
  order: 1;
  background: blue;
}
.c {
  background: red;
}

Quick codepen sample with -webkit- prefixes

UPDATE! Tuned pen with landscape/portrait ratios