<div class="items">
    
    <div class="items__item">5</div>
    <div class="items__item">5</div>
    <div class="items__item">5</div>
    <div class="items__item">5</div>
    <div class="items__item">5</div>

    <div class="items__item">4</div>
    <div class="items__item">4</div>
    <div class="items__item">4</div>
    <div class="items__item">4</div>

    <div class="items__item">3</div>
    <div class="items__item">3</div>
    <div class="items__item">3</div>
    
</div>

.items {
  display: flex;
  flex-wrap: wrap;
  text-align: center;
}

.items__item {
  flex: 1 1 20%;
}

.items__item:nth-child(6), 
.items__item:nth-child(7), 
.items__item:nth-child(8), 
.items__item:nth-child(9) {
  flex-basis: 25%;
}

.items__item:nth-child(10), 
.items__item:nth-child(11), 
.items__item:nth-child(12) {
  flex-basis: 33.3333%;
}