Demo: https://codepen.io/P42s/pen/RwLKeep

<div class="container">
	<div class="cell cell-1">
		<h2>A</h2>
	</div>
	<div class="cell cell-2">
		<h2>B</h2>
	</div>
	<div class="cell cell-3">
		<h2>C</h2>
	</div>
</div>

.container {
	display: flex;
	flex-direction: column;
	flex-wrap: wrap;
	height: 500px;
	gap: 20px;
}

.cell {
	display: flex;
	align-items: center;
	justify-content: center;
	background-color: #eee;
	border: solid 3px #000;
	border-radius: 12px;
	width: 50%;
	padding: 50px;
	flex: 1;
}

.cell-3 {
	flex-basis: 100%;
}

@media (max-width: 767px) {
	.container {
		flex-wrap: nowrap;
	}
	.cell {
		width: 100%;
	}
	.cell-1 {
		order: 1;
	}
	.cell-2 {
		order: 3;
	}
	.cell-3 {
		order: 2;
	}
}