Achaa, you only mentioned disabling cores - not enabling them.
There is about a 75% chance you can unlock a dual-core Phenom II to a quad-core Phenom II, and about a 90% chance you can unlock it to a tri-core Phenom II, and then you need a decent BIOS to do so. Some manufacturers support this core unlocking thing officially, but some don't. Either way, most BIOS's will allow you to try to unlock the cores.
However, if things get unstable after overclocking the other two cores, you'll know your processor fell into that 25% which aren't able to unlock the other two cores.