Are you sure it has 3GB? Apple only shipped macs with 32-bit Intel chips very briefly (in 2006, with the Core Duo, which is i686) and I think they couldn’t have more than 2GB ram. Maybe you have a later model which actually has a 64-bit Core 2 Duo but the 64-bit Debian installer just didn’t work on it.
Are you sure it has 3GB? Apple only shipped macs with 32-bit Intel chips very briefly (in 2006, with the Core Duo, which is i686) and I think they couldn’t have more than 2GB ram. Maybe you have a later model which actually has a 64-bit Core 2 Duo but the 64-bit Debian installer just didn’t work on it.