How do i change my desktop backround to change every 3 hours?

Hi Ebony_Greene,

The steps here cover your options for automatically changing your Wallpaper with intervals:

Choose a time interval from the pop-up menu to specify how frequently wallpaper pictures and colours change when you set them to automatically cycle (see “Auto-Rotate”, below).

Cycle through the pictures or colours in order or select Randomly.

This option may affect the performance of some apps, such as games, that use many graphics.

You can find these steps along with other information that may be helpful, here: Change Wallpaper settings on Mac – Apple Support

If you do not see an interval that suites you, we are always looking for ways to improve our products, so we encourage you to leave feedback here: Product Feedback - Apple

This is the best way to have your voice heard by our designers and engineers.

Cheers!