The Steam Link app is now available on the Raspberry Pi 3B and 3B+ running Raspbian Stretch. A wired network is strongly recommended.
To install, run the following from a command line:
sudo apt update
sudo apt install steamlink
Or download and install directly from Valve:
curl -#Of http://media.steampowered.com/steamlink/rpi/steamlink.deb
sudo dpkg -i steamlink.deb
To run, you can launch it from the Games menu, or run the following from a command line:
steamlink
Support and troubleshooting are available here:
https://steamcommunity.com/app/353380/discussions/6/1743354432807054634/
This is saved from: https://steamcommunity.com/app/353380/discussions/6/2806204039992195182/