I would take a guess and say you have to pair every time you switch device. the PS3 controller dont support multiple device at a time.I know you can with Sixaxis (in the Play store).
My main question is one that I've not found a definite answer for yet... can a PS3 controller be paired to more than just one device ? ie; if I paired it to my N7 via Sixaxis, can I also use it for my PS3 ? Or do I need to keep pairing it to each device I want to use it on each time ?