This is Bootstrap v4, so the col-4 is correct… col-md-4 is Bootstrap 3.
ETA! Sorry, col-md-4 is correct in v4 too… but thats not whats causing the problem, col-4 is still fine…its the card div.
@rainythunderstorm The div class card is throwing things off by messing with the built in bootstrap margin settings… by giving each card a fixed margin of 20px for a total of 40px per card, that is going to affect how your columns adjust down to mobile view.
Remove the card div, and make your adjustments, in percentages on the col-4 class.