Just add col-md-12 (breakpoint of 768px or higher will be full width).
Then, add col-4 as well, that will affect from this breakpoint down.

<div class="row">
    <div class="col-4 col-md-12"></div>
    <div class="col-4 col-md-12"></div>
    <div class="col-4 col-md-12"></div>
</div>

You can see more in the Bootstrap documentation how this grid system works.

Using bootstrap, with this sample :

<div class="row">
    <div class="col-md-4"></div>
    <div class="col-md-4"></div>
    <div class="col-md-4"></div>
</div>

After passing the breakpoint of 768px your cols will go from a vertical alignment to an horizontal alignment. How can I reverse this effect ? I need my cols to start horizontal then go vertical after the breakpoint.