HTML
div class="container">
div class="first">
div class="second">
div class="third">
container /div
CSS
.container{
display: flex;
flex-direction: row;
}
.first, .second, .third{
margin-right:10px;
padding-top: 10px;
width: 400px;
height: 300px;
background-color: black;
color: white;
text-align: center;
}