Judging by his profile (UK), Shift-3 for him is probably Pound. Unfortunately, that means something different in the UK than it does in the US, though for reasons not unrelated. UK keyboards have a separate key for #.

Ah I didn't realise it was region specific. Yeah the alternative item underneath the 3 button is the symbol for pound sterling (£), the # symbol is a hidden extra.

"Pound" as in "Pounds Sterling" not "Pound" as in "Oglethorpe" I take it?