Why do my boxes not line up by 3 as anticipated? [un-equal height divs]

Fix your code errors.

You only need to express  columns for one breakpoint, all the rest will be the same.   Keep in mind that 3 columns probably won't fit on small devices.   So in this example there will be 1 column on xs and sm, 3 columns on md, lg, and xlg.

<div class="container">

<div class="row">

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

Something here....

</div>

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

Something here....

</div>

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

Something here....

</div>

</div>

</div>