<div class="grid">
  <div class="grid__item">1</div>
  <div class="grid__item">2</div>
  <div class="grid__item">3</div>
  <div class="grid__item">4</div>
  <div class="grid__item">5</div>
  <div class="grid__item">6</div>
  <div class="grid__item">7</div>
  <div class="grid__item">8</div>
  <div class="grid__item">9</div>
  <div class="grid__item">10</div>
  <div class="grid__item">11</div>
  <div class="grid__item">12</div>
  <div class="grid__item">13</div>
  <div class="grid__item">14</div>
  <div class="grid__item">15</div>
</div>

.grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-auto-flow: dense;
  gap: 4px;
  text-align: center;
}

.grid__item {
  padding: 4px;
  outline: 1px solid lightblue;
}

.grid__item:nth-child(6n + 1),
.grid__item:nth-child(6n + 2) {
  grid-column: 1 / 2;
}

.grid__item:nth-child(6n + 3),
.grid__item:nth-child(6n + 4) {
  grid-column: 2 / 3;
}