Tutorial: https://www.youtube.com/watch?v=GwSf9KGLn5w

<style>

/* Customize every 1st, 2nd, & 3rd collection item itself */
.your-collection-item:nth-child(3n+1) {background-color: red;}
.your-collection-item:nth-child(3n+2) {background-color: blue;}
.your-collection-item:nth-child(3n+3) {background-color: green;}

/* Customize every 1st, 2nd, & 3rd item inside your collection item */
.your-collection-item:nth-child(3n+1) .item-inside-collection-item {background-color: yellow;}
.your-collection-item:nth-child(3n+2) .item-inside-collection-item {background-color: green;}
.your-collection-item:nth-child(3n+3) .item-inside-collection-item {background-color: purple;}

</style>