Custom JavaScript for Websites 3 is a Chrome add-on that allows users to run custom JavaScript on any website. With this tool, users can inject their own JavaScript code into websites for various purposes. The extension supports the Chrome sync API, ensuring that the custom scripts are saved and applied across domain URLs.

One of the key features of Custom JavaScript for Websites 3 is the ability to inject custom JavaScript based on domain or custom Regular Expression. This provides users with flexibility in targeting specific websites or web pages for their custom code.

The add-on also includes a dedicated tab with an Ace Editor, which offers formatting, highlighting, and undo/redo functionalities for easier code editing. The extension also supports draft auto save, ensuring that users don't lose their work even if they accidentally close the window without saving.

Custom JavaScript for Websites 3 also allows users to include external scripts, such as jQuery and MooTools, or their own external scripts. This opens up possibilities for site debugging, extracting useful information from websites, hiding annoying popups and ads, creating custom UI, and more.

Overall, Custom JavaScript for Websites 3 provides a powerful tool for users to customize their browsing experience and enhance website functionality with custom JavaScript code.