If I get you right, you want something that seems to be the opposite of what is desired normally: you want a horizontal layout for small screens and vertically stacked elements on large screens. You may achieve this in a way like this:
<div class="container">
<div class="row">
<div class="hidden-md hidden-lg col-xs-3 col-xs-offset-6">a</div>
<div class="hidden-md hidden-lg col-xs-3">b</div>
</div>
<div class="row">
<div class="hidden-xs hidden-sm">c</div>
</div>
</div>
On small screens, i.e. xs and sm, this generates one row with two columns with an offset of 6. On larger screens, i.e. md and lg, it generates two vertically stacked elements in full width (12 columns).