SOURCE

<div class="container">
	<div class="left">
		
	</div>
	<div class="content"></div>
	<div class="right"></div>
</div>

.container div {
	height:100px;
}
.container {
	display: flex;
}
.left {
	background-color: tomato;
	width: 50px;
}
.right {
	width: 50px;
	background-color: pink;
}
.content {
	flex-grow: 1;
	background-color: black;
}