hp 24 mh monitors go black when in full screen mode

Hello SJSH1,

Welcome to Apple Support Communities. This may be due to your display settings. In order to use apps in split view, your displays must have separate spaces. To change your display preferences, go to Apple menu  > System Preferences, then click Mission Control. Check out Change Mission Control preferences on Mac - Apple Support for details about each setting.

Take care.

Hi SJSH1,

Did you logout after checking the settings recommended by faith185? If not, test after a as the feature should require a logout of the account to apply the settings.

If the issue remains, have you double checked to see how many displays are supported by your MacBook Pro? The article here: If your external display isn't detected or appears blurry when connected to your Mac and steps quoted below can assist.

"Check how many displays you can connect

If you're connecting multiple displays to your Mac, use the Tech Specs page to find out how many external displays your Mac supports.

1. Choose Apple menu  > About This Mac, double-click your serial number, then press Command-C to copy it.
2. Go to the Tech Specs page, click the Search Tech Specs bar, press Command-V to enter your computer's serial number, then click Search.
3. Click the specs page for your Mac. The number of displays your Mac supports appears under Video Support or Graphics."

If your hardware supports three monitors and the issue remains, reach out to Apple Support directly to further assist.

Contact - Official Apple Support

Contact Apple for support and service - Apple Support

Cheers!

<< When I put anything in full screen mode, the other monitor plus my MacBook monitor go black. >>

"Full Screen" is a feature reverse ported from the iPhone, land of tiny screens and one-at-a-time processing. In my opinion, it is brain-damaged.

If you would like to use a window that nearly engulfs one display, I recommend you stretch the window to the size of the display, which will not cause the second display to go dark.

"Extended Desktop" is the Mac feature for multiple displays:

.

as long as you don't invoke "Full Screen", you can continue to expand the window onto the next display as well. When you scroll the window, the parts on different displays will scroll "the right way".