I usually use my phone power brick (rated at 5v 2.5Amps) to run my Pi 3 and it works fine for the most part.
Now I bought a new phone which supports something called fast charge. So the power brick that came with it has 3 output ratings. Its written like 5v 3Amps or 9v 1.2Amps or 12v 1Amp on the back of my PSU.
Shall I use this PSU with my Pi or do I just keep on using the one I've been using?