Rock64 Cluster.

[fire219]

Any PSU would work, as long as it meets a few requirements:

• Stable 4.9-5.2V output. Don't just trust it if it says 5V. It may put out much less under heavy load and much more under minimal load.
  
• 3 amps per board. If you are running 3 boards, the power supply needs to be rated for at least 9 amps. More is fine, and is even preferable.
  
• Good cables/wire between power supply and boards. Wires that are too thin may make the voltage drop too low, and might even heat up and burn.
  

You'll also need to decide on if you want to power the boards through the barrel jack connector, or through the pin header. You'll probably have to do your own wiring, since power supplies that fit your need will be like this.