Полагаю, это то, что вы ищете:
Кажется, что вы настроили размер m-0 mx-md-5 col-sm-12 col-md-4 d-block mb-3. Поэтому я просто использовал их повторно (я думаю, вам не нужно так много занятий, чтобы это работало, это не хорошо для эффективности).
В любом случае, вы отвечаете:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>movieDB</title>
<link rel="stylesheet" href="ps4.css">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css" integrity="sha384-Vkoo8x4CGsO3+Hhxv8T/Q5PaXtkKtu6ug5TOeNV6gBiFeWPGFN9MuhOf23Q9Ifjh" crossorigin="anonymous">
</head>
<body>
<nav class="navbar navbar-expand-lg navbar-light bg-light">
<a class="navbar-brand" href="#">MovieDB</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav mr-auto">
<li class="nav-item">
<a class="nav-link" href="#">Credits</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Filter
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Comedy</a>
<a class="dropdown-item" href="#">Fantasy</a>
<a class="dropdown-item" href="#">Kids</a>
<a class="dropdown-item" href="#">pg</a>
</div>
</li>
</ul>
<form class="form-inline my-2 my-lg-0">
<input class="form-control mr-sm-2" type="search" placeholder="Search Movie" aria-label="Search">
<button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
</form>
</div>
</nav>
<h1 class="display-1 d-flex justify-content-center mb-5"><img src="https://www.freelogodesign.org/file/app/client/thumb/da9eb726-0742-4b5b-a19a-7c42fcaa54ec_200x200.png?1581566782305" alt=""> </h1>
<div class="m-0 mx-md-5 col-sm-12 col-md-4 d-block mb-3">
</div>
<img class="m-0 mx-md-5 col-sm-12 col-md-4 d-block mb-3" src="https://i.ebayimg.com/images/g/LVEAAOSwIttddvIz/s-l400.jpg" alt="">
<div class="m-0 mx-md-5 col-sm-12 col-md-4 d-block mb-3">
<div class="text-center" style="width: 100%; height: 100%;">
<button class="btn btn-outline-danger ml-5 p-2 ml-md-5" style="
margin: 0!important;" type="submit">❤</button>
<h3 class="d-inline border border-success rounded-pill px-5">DB: 7.9</h3>
<button class="btn btn-outline-danger mr-5" style="
margin: 0!important;" type="submit">Search</button>
</div>
</div>
<script src="https://code.jquery.com/jquery-3.4.1.slim.min.js" integrity="sha384-J6qa4849blE2+poT4WnyKhv5vZF5SrPo0iEjwBvKU7imGFAV0wwj1yYfoRSJoZ+n" crossorigin="anonymous"></script>
<script src="https://cdn.jsdelivr.net/npm/popper.js@1.16.0/dist/umd/popper.min.js" integrity="sha384-Q6E9RHvbIyZFJoft+2mJbHaEWldlvI9IOYy5n3zV9zzTtmI3UksdQRVvoxMfooAo" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/js/bootstrap.min.js" integrity="sha384-wfSDF2E50Y2D1uUdj0O3uMBJnjuUD4Ih7YwaYd1iqfktj0Uod8GCExl3Og8ifwB6" crossorigin></script>
</body>
</html>