%h3 Первый вариант
%ol.v1
  - (1..4).each do
    %li
%ol.v1
  - (1..5).each do
    %li
%ol.v1
  - (1..6).each do
    %li

%h3 Второй вариант
%ol.v2
  - (1..4).each do
    %li
%ol.v2
  - (1..5).each do
    %li
%ol.v2
  - (1..6).each do
    %li

ol {
  display: inline flex;
  width: 160px;
  margin: 0;
  padding: 0;
  flex-wrap: wrap;
}

li {
  list-style: inside decimal;
  font-size: 16px;
  padding: 2px;
  aspect-ratio: 1;
  flex: 45px 0 0;
  background: #b5e61d;
  box-sizing: border-box;
  margin: 2px;
}

.v1 li:nth-child(3n + 1):nth-last-child(-n + 2),
.v1 li:nth-child(3n + 1):nth-last-child(-n + 2) ~ li {
  background: red;
}

.v2 li:nth-child(3n):nth-last-child(-n + 3) ~ li {
  background: cyan;
}