I am trying to create a simple responsive three level menu in Bootstrap 4 using only bs4, html and css. Surprisingly for me it appears there is no submenu class in bs4 which script does not use js. So i tried to use some external css and created an <ul> in the file, but there are still obvious problems to achieve the goal. It does not work : ( Could you, please, give me some clues and hints what i am doing wrong? I imagine it is something simple and stupid. Thank you!

p.s. My first post in the forum, I am not quite sure how to correctly edit my code. I wrapped it in html tag, i hope it looks ok.

HTML Code:
ul{
	list-style:none;
	padding:0px;
	margin:0px;
}


nav li .dropdown-menu:hover .child {
	display:block;
}

.navbar {
	position:relative;
}

.dropdown-menu{
	position:absolute;
	top:100%;
	left:0px;
}

.child{
	position:absolute;
	top:40px;
	right:100%;
	display:none;
	border:1px solid black;
	padding:10px;	
}

.child li a{
	color:#000;
	text-decoration:none;
}

<body>
<nav class="navbar navbar-expand-md bg-dark navbar-dark">
<a class="navbar-brand" href="#">Logo</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#collapsibleNavbar">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="collapsibleNavbar">
<ul class="navbar-nav">
<li class="nav-item">
<a class="nav-link" href="#">Home</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">About us</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" data-toggle="dropdown" id="dropdown" aria-haspopup="true" aria-expanded="false" href="#">Cars</a>
<div class="dropdown-menu" aria-labelledby="#dropdown">
<a class="dropdown-item" href="#">Opel</a>	
<a class="dropdown-item" >Volvo</a>
<ul class="child">
<li><a href="#">V50</a></li>
<li><a href="#">S40</a></li>
<li><a href="#">XC90</a></li>
</ul> 
<a class="dropdown-item" href="#">Renault</a>
</div> 
</li>
<li class="nav-item">
<a class="nav-link" href="#">Contacts</a>	
</li>
</ul>
</div> 
</nav>
<br>
</body>