I used google and searched download 3cx version 12 and found this link -
http://downloads.3cx.com/downloads/3CXPhoneSystem12.exe
3CX only supports the current and 1 prior versions which would be v14 and v12.5. I would recommend you upgrade your system. Of course if you are running 32 bit Windows you will need to upgrade to 64 bit Windows - this should be a free upgrade.
Regards,
Allen