how should i assign flex end for inline flex code example

Example: whats the best ways to 3 x 3 perfects squares on a webpage using flexbox

.parent {
  display: flex;
  height: 300px; /* Or whatever */
}

.child {
  width: 100px;  /* Or whatever */
  height: 100px; /* Or whatever */
  margin: auto;  /* Magic! */
}