Ummm... it's right there on your keyboard. Just hold down shift, and press 3... I hope I didn't misunderstand you.
Note the "UK" comment...
Dynamicv's is probably the easiest (and standard) way to do it. Otherwise, you can add one of two things to the menu bar:
- Go to system preferences -> International -> Input Menu -> Show Input Menu in menu bar
And then select one/both of the Character Palette and Keyboard Viewer from the menu. Then you can launch these from the menu bar system-wide. The palette functions like the Insert Symbol option in MS Word. The Kbd viewer shows you what the keyboard layout is, including for instance, if you hold shift, it will show you what each key makes with shift, if you hold option, etc.