RaspberryPi.org puts it this way: "We’re recommending a 2.5A adapter if you want to connect power-hungry USB devices to the Raspberry Pi".
I decided a power supply with 5 V and 2.0 A has to do since that's the most powerful power I own. Only keyboard and mouse are attached. And yes, no need to find wifi dongle anymore!
Raspi 3 booted and is running Raspbian at the moment. I think power supply with 5 V and 2,0 A is enough for now.
Raspi 3 has built-in wifi!