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
To run, you can launch it from the Games menu, or run the following from a command line:
steamlink
For troubleshooting and feedback, please visit the Steam Link Raspberry Pi Discussions