+---------------------------+
|  A  |       B       |  C  |
+---------------------------+

+---------------+
|   A   |   C   |
|       B       |
+---------------+

  
A
  
B
  
C

* {
  box-sizing: border-box;
}
body, html {
  margin: 0;
  padding: 0;
}
.content {
  display: flex;
  flex-wrap: wrap;
}

.a, .b, .c {
  border: 1px solid black;
  flex: 1;
  padding: 10px;
}

@media(max-width: 768px) {
  .b {
    flex: 0 0 100%;
    order: 2;
  }
}