How to get button groups that span the full width of a parent in Bootstrap?

Correct solution for Bootstrap 4 and Bootstrap 5 (from Bootstrap 4 migration documentation):

Removed .btn-group-justified. As a replacement you can use <div class="btn-group d-flex" role="group"></div> as a wrapper around elements with .w-100.

Example:

<div class="btn-group d-flex" role="group" aria-label="...">
  <button type="button" class="btn btn-default w-100">Left</button>
  <button type="button" class="btn btn-default w-100">Middle</button>
  <button type="button" class="btn btn-default w-100">Right</button>
</div>

Source: https://getbootstrap.com/docs/4.0/migration/#button-group

Edit: verified solution with Bootstrap 5

You can also use bootstraps btn-block will span across parent.

Create block level buttons—those that span the full width of a parent—by adding .btn-block.

<button type="button" class="btn btn-primary btn-lg btn-block">Block level button</button>
<button type="button" class="btn btn-secondary btn-lg btn-block">Block level button</button>

https://getbootstrap.com/docs/4.0/components/buttons/

Flexbox can do that.

.btn-group.special {
  display: flex;
}

.special .btn {
  flex: 1
}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="stylesheet" />
<div class="btn-group special" role="group" aria-label="...">
  <button type="button" class="btn btn-default">Left</button>
  <button type="button" class="btn btn-default">Middle</button>
  <button type="button" class="btn btn-default">Right</button>
</div>