div > div:nth-child(3n) + div:nth-last-child(1) {
  border-left: 5px solid black;
  border-right: 5px solid black;
}
div > div:nth-child(3n+1) + div:nth-last-child(1) {
  border-left: 5px solid black;
}
div > div:nth-child(3n+1) + div:nth-last-child(2) {
  border-left: 5px solid black;
  border-right: 5px solid black;
}

div > div:nth-child(3n) + div:nth-last-child(1)

div > div:nth-child(3n) + div:nth-last-child(1)

div > div:nth-child(3n+1) + div:nth-last-child(2)

div > div:nth-child(3n+1):nth-last-child(2)