How do I share what is happening on my HTC VIVE Focus 3 with others on a screen?

This is called screen casting. The Focus 3 supports this functionality.

The instructions are available on this page:  https://www.vive.com/ca/support/focus3/category_howto/casting-your-screen.html

 

Here are the instructions for casting to a PC:  https://www.vive.com/ca/support/focus3/category_howto/casting-to-pc.html