/* with flexbox */
.col54-masonry-holder {
  display: -webkit-box;
  display: -webkit-flex;
  display: -ms-flexbox;
  display: flex;

  gap: 60px;
}

.col54-masonry-col {
  -webkit-box-flex: 1;
  -webkit-flex-grow: 1;
  -ms-flex-positive: 1;
  flex-grow: 1;
  flex-basis: 0;
}

/* 2 columns by default, hide columns 2 & 3 */
.col54-masonry-col-2, .col54-masonry-col-3 { display: none }

/* 3 columns at medium size */
@media ( min-width: 768px ) {
  .col54-masonry-col-2 { display: block; } /* show column 2 */
}

/* 4 columns at large size */
@media ( min-width: 1080px ) {
  .col54-masonry-col-3 { display: block; } /* show column 3 */
}