You must disable the flexbox for larger widths because Bootstrap 4’s flexbox makes columns the same height. Then, use floats so that the B & C columns naturally pull to the right, since A is taller. The flexbox order-
will work to reorder the cols on mobile…
A-B-C desktop, B-A-C mobile
<div class="container-fluid">
<div class="row d-flex d-lg-block">
<div class="col-lg-6 order-1 float-left">
A
</div>
<div class="col-lg-6 order-0 float-left">
B
</div>
<div class="col-lg-6 order-1 float-left">
C
</div>
</div>
</div>