html
                 <div id="main">
    <div id="c1"></div>
    <div id="c2"></div>
    <div id="c3"></div>
</div>
 
            

        
css
                 #main {
    display: flex;
}

#c1 {
    background-color: red;
    flex: 1;
    height: 200px;
}

#c2 {
    background-color: blue;
    flex: 1;
}
#c3 {
    background-color: green;
    flex: 1;
}

 
            

            
js