Demo

style.css
setup.css
index.html

section {
	display: grid;
	grid-template-columns: repeat(3, 1fr);
	grid-template-rows: 100px 50vh 100px;
	grid-template-areas:
		"header header header"
		"main   main   aside"
		"footer footer aside";
}

header {
	background-color: crimson;
	grid-area: header;
}

main {
	background-color: hotpink;
	grid-area: main;
}

aside {
	background-color: lightsalmon;
	grid-area: aside;
}

footer {
	grid-area: footer;
}

body {
	font-family: sans-serif;
	padding: 20px;
}

section {
	background-color: gold;
}

section > * {
	background-color: tomato;
	border-bottom: 2px solid firebrick;
	border-right: 2px solid firebrick;
	padding: 5px;
}

<!DOCTYPE html>
<html>
	<head>
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<link href="/assets/styles/reset.css" rel="stylesheet">
		<link href="setup.css" rel="stylesheet">
		<link href="style.css" rel="stylesheet">
	</head>
	<body>
		<section>
			<header>
				<p>Item 1 will span across the top</p>
			</header>
			<main>
				<p>Item 2 with a lot of text in it that takes up some space, again I am writing this manually for some reason, but we’re already here so I may as well just keep writing nonsense to increase the height and show the layout better</p>
			</main>
			<aside>
				<p>Item 3 continues down the side</p>
			</aside>
			<footer>
				<p>Item 4 might also have enough text to wrap and add height</p>
			</footer>
		</section>
	</body>
</html>