:root {
  --columns: 1;
  --item-width: calc(100% / var(--columns));
  --gutter: 20px; /* only px will work */
}

body {
  font: normal 125%/1.4 Georgia, 'Times New Roman', Times, serif;
  max-width: 1200px;
  padding: 2ch;
  margin: auto;
  text-align: center;
  color: #222430;
}

p {
  max-width: 60ch;
  margin: 20px auto;
}

a {
  color: #d34a97;
}

.container {
  display: flex;
  flex-flow: column wrap;
  align-content: space-between;
  background-color: #f7f7f7;
  border-radius: 3px;
  padding: 20px;
  margin: 40px auto;
  counter-reset: items;
}

div.item::before {
  counter-increment: items;
  content: counter(items);
}

.item {
  width: var(--item-width);
  margin-bottom: calc(var(--gutter) * 2);
  box-sizing: border-box;
  position: relative;
  border-radius: 3px;
  background-color: #b1d6c2;
  border: 1px solid #7c9788;
  box-shadow: 0 2px 2px rgba(0,90,250,0.05),
    0 4px 4px rgba(0,90,250,0.05),
    0 8px 8px rgba(0,90,250,0.05),
    0 16px 16px rgba(0,90,250,0.05);
  color: #fff;
  padding: 15px;
}

.container::before,
.container::after {
  content: "";
  flex-basis: 100%;
  width: 0;
  order: 2;
  display: none;
}

@media (min-width: 600px) {
  :root {
    --columns: 2;
    --item-width: calc(100% / var(--columns) - var(--gutter));
  }

  .item:nth-of-type(2n+1) { order: 1; }
  .item:nth-of-type(2n) { order: 2; }
  .container::before { display: block; }
}

@media (min-width: 1000px) {
  :root {
    --columns: 3;
  }

  .item:nth-of-type(3n+1) { order: 1; }
  .item:nth-of-type(3n+2) { order: 2; }
  .item:nth-of-type(3n) { order: 3; }
  .container::after { display: block; }
}