Note: The M3 Pro or M3 Max chip supports connecting up to four external monitors. The standard M3 chip only supports a single external monitor.

Check the display mode
Go to System Preferences, click Displays, and then click Arrangement. Make sure the Mirror Displays box is not checked. Also, confirm that the display mode is set to Extend these displays. 

Check the connection
Make sure the cable is securely connected to the correct port on your Mac. You can also try unplugging and replugging the display. 

Connect directly to your Mac
If your display is connected to a hub or dock, try connecting it directly to your Mac. 

Use a different cable
Make sure you're using the correct cable to connect your display. 

Use a display link
You can use a display link chip and driver to create virtual graphics cards that allow you to extend your desktop across multiple screens.