https://tobiasahlin.com/blog/masonry-with-css/

mansory.css

.container {
  display: flex;
  flex-flow: column wrap;
  align-content: space-between;
  /* Your container needs a fixed height, and it 
   * needs to be taller than your tallest column. */
  height: 600px; 
}

.item {
  width: 32%;
  margin-bottom: 2%; /* Optional */
}

/* Re-order items into 3 rows */
.item:nth-child(3n+1) { order: 1; }
.item:nth-child(3n+2) { order: 2; }
.item:nth-child(3n)   { order: 3; }

/* Force new columns */
.container::before,
.container::after {
  content: "";
  flex-basis: 100%;
  width: 0;
  order: 2;
}