Ok so I dont know anything about cell phone batteries, so I could be entirely wrong, but here are some things you might want to look into.
Laptops use 'smart batteries,' meaning that they have built in electronics to tell your laptop how much charge it has, whether it is broken, and many other things. Considering how rapid cell phone technology (and battery technology) is changing, you may have to hack the smart part of the battery if it has one. This could quite possibly be what the 3rd wire is about . . .
Let us know what you learn if/when you figure it out, I'm really curious now . . .