You don't "register" a PS3 controller in the traditional sense. The PS3 automatically recognizes and pairs with any compatible controller connected to it.
However, you might be referring to these situations:
1. Connecting a New Controller for the First Time
Ensure PS3 is turned on.
Connect the controller via USB cable to pair it.
Once paired, disconnect the USB cable for wireless use.
2. Connecting a Controller Previously Paired with Another Console