Alt-3 for # working fine here (in the UK). You might have non-standard shortcuts set up in System Prefs > Mission Control or an odd keyboard layout installed.
What?
Alt-3 works just fine me on a UK Mac for making a #. I have made no tweaks.
What OS are you running? What are your physical keyboard and keyboard map, UK or US?
Mavericks, but behaviour has stayed consistent since OS X Public Beta (pre-Cheetah) for me and was probably the same on Classic.
Physical keyboards I've used have always been en_GB, internal or external.
Since Lion Apple has decided that just making our lives difficult wasn't enough, we really don't deserve the # symbol at all, and now that combination switches desktops. There is no keyboard combination on a standard, un-hacked factory delivered UK Mac that produces a # symbol. Really.
I keep a text file on my desktop that contains a single # character and cut-n-paste it when I need one.