Using CSS only:
#blockContainer {
display: -webkit-box;
display: -moz-box;
display: box;
-webkit-box-orient: vertical;
-moz-box-orient: vertical;
box-orient: vertical;
}
#blockA {
-webkit-box-ordinal-group: 2;
-moz-box-ordinal-group: 2;
box-ordinal-group: 2;
}
#blockB {
-webkit-box-ordinal-group: 3;
-moz-box-ordinal-group: 3;
box-ordinal-group: 3;
}
<div id="blockContainer">
<div id="blockA">Block A</div>
<div id="blockB">Block B</div>
<div id="blockC">Block C</div>
</div>