You can either put the classes on the "row" or the "column"? Won't be visible on the cards (border) if you use it on the row. https://v4-alpha.getbootstrap.com/utilities/flexbox/#align-items
<div class="container">
<div class="row">
<div class="col-lg-4 d-flex align-items-stretch">
Some of the other answers here seem 'wacky'. Why would use a min-height or even worse a fixed height?
I thought this issue (equal heights) was part of the reason for progression away from floating block divs?