How do I turn on two cameras and a screen demo at the same time?

Is your teacher familiar with this workflow or are they asking for something they don't actually understand?

The short answer is that Zoom only supports 2 streams from a given Workplace/Client connection - 1 for camera, 1 for content (a second camera can be shared as content). Based on your description, your teacher is asking for (3) streams - camera1, camera2, and content.

There are, of course, plenty of workarounds such as:

• Using a second connection (different user) for another camera or content-share
• Compositing multiple cameras using software like OBS, vMix, Manycam, etc
• Using a Zoom Room and turning on multiple cameras while sharing content