<!-- Topバナー -->
div{
   margin-bottom:15px;
}
  .parent {
   max-width: 100%;
   display: flex;
   flex-wrap: wrap; 
 }

  .child {
   width:32%;
   padding:2px;
 }

 @media screen and (max-width: 1200px) {
   .child {
     width:45%;
   }
 }

 @media screen and (max-width: 800px) {
   .child {
     width:100%;
     height:calc(100vw/1.835);
   }
 }
<!-- Topバナー -->