Shift Command 3 not working as of today

What do you see in System Preferences->Keyboard->Shortcuts->Screenshots?

If the command-shift-3 is there and checked, try unchecking, then quit System Preferences, and then go back and check it again.

If Luis' suggestion doesn't fix the problem boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and see if the problem persists. Reboot normally and check again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Evelynneedinghelp wrote:

I'm trying to find com.apple.symbolichotkeys.plist/Users/YourName/Library/Preferences

in my preferences folder to delete it, but it isn't there.

From Finder>Go>Go To Folder copy and paste:

~/Library/Preferences/com.apple.symbolichotkeys.plist

What do you see in System Preferences->Keyboard->Shortcuts->Screenshots?

What happens when you press Command-Shift-5? Does that work?

What happens when you press Command-Shift-4? Does that work?

I don't have an Apple extended keyboard (or what they are called now).

I read that you may have to press Clear, or sometimes Shift-Clear, depending on keyboard settings.

Do you see something about the keypad keys in System Preferences->Keyboard->Keyboard? The appearance of this varies with mac model and which keyboard is being used. As I said, I don't have a similar keyboard, so I can't check how that looks, but maybe you can find something there.

Have you accidentally enabled Mouse Keys?

System Preferences->Accessibility->Pointer Control->Alternative Control Methods and make sure that "Enable Mouse Keys" is NOT checked.

Do the number keys otherwise work, e.g., if while typing text you press 3 in the numeric keypad does it work?