how to get every 3 childs last one in css code example

Example 1: css last 3 childs

#something a:nth-last-child(-n+3) {
    /*declarations*/
}

Example 2: nth-child(2n+1)

/* Select the first list item */
li:nth-child(1) { }

/* Select the 5th list item */
li:nth-child(5) { }

/* Select every other list item starting with first */
li:nth-child(odd) { }

/* Select every 3rd list item starting with first */
li:nth-child(3n) { }

/* Select every 3rd list item starting with 2nd */
li:nth-child(3n - 1) { }

/* Select every 3rd child item, as long as it has class "el" */
.el:nth-child(3) { }