The "64-bit" in Boot Camp 3.0 refers to the drivers required to run 64-bit versions of Windows (specifically Windows 7 and later Vista/8).
Step 6 — Install Windows (64-bit)
Use the open-source tool Brigadier to fetch Boot Camp drivers directly from Apple’s servers: boot camp 3.0 64 bit
To use Boot Camp 3.0 64-bit, users need to meet the following system requirements: The "64-bit" in Boot Camp 3