CSS nth-3n is a property that allows you to specify the third iteration of a specific CSS class. It is useful when you want to apply a set of rules to a specific element in a sequence, such as in a loop or a repeating pattern.

The syntax for using CSS nth-3n is as follows:

```css

element:nth-child(3n) {

/* CSS rules for the 3rd iteration of the element */

Here, `element` is the name of the element you want to apply the rules to. The `3n` refers to thenth-child(3), which means the 3rd iteration of the specified class.

By default, the `nth-child` property selects the 1st, 2nd, 4th, and 5th iteration of a class, but it can be changed to select any other iteration by using the `3n` syntax.

For example, if you want to select the 3rd, 4th, and 5th iteration of a class on the first, second, and third elements in a loop, you can use the following code:

```css

HTML

• Item 1
• Item 2
• Item 3
• Item 4

CSS

In this example, the first element will have a red background color, the second element will have a blue background color, the third element will have a green background color, and the fourth element will have a red background color.

Overall, CSS nth-3n is a useful property that allows you to specify the third iteration of a specific CSS class and apply rules to specific elements in a sequence.