keyboard not showing pound

What language is your keyboard set to? On a UK keyboard Shift 3 gives the £ sign.

Go to System Preferences>Keyboard>Input Sources, tick 'Show input menu in menu bar'. From that icon in the menu bar select 'Show keyboard viewer' and it should show you where you can find the £ sign when you press the shift key and/or the option (alt) key.

If you have a US keyboard, it is at option/alt + 3.

thanks for that ! it is as you say on my keyboard alt + 3