To do Alt + 3 on Mac in PowerPoint, download and install Accelerator Keys, then press Option, followed by 3.
Command: Quick Access Position 3 (user-customizable)
This is how to do Alt + 3 on Mac in PowerPoint with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with PowerPoint.
Alt + 3 activates the third button in your Quick Access Toolbar. By default, this is typically Redo, but you can customize it to any command you frequently use. The Quick Access Toolbar appears at the top-left of PowerPoint, making these shortcuts some of the fastest ways to access your most-used features.