even this will work:
<div class="container">
<div class="row">
<div class="col-4 col-sm-4 col-md-6 order-1">
1
</div>
<div class="col-4 col-sm-4 col-md-6 order-3">
2
</div>
<div class="col-4 col-sm-4 col-md-12 order-2">
3
</div>
</div>
</div>