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This page shows you the various remotes currently available from Roku. The Roku Voice models previously mentioned in other replies connect to the Roku via WiFi and so are point-anywhere. The Roku Players remote (formerly called the Simple remote) and Roku TV remote control via infrared (IR) and must be pointed in the direction of the Roku they are to control.
Your Roku 3 came with a WiFi remote but also can be controlled by IR. It is compatible with all Roku remotes except the Roku TV remote (Roku TVs use a different set of IR control codes from those for separate Roku devices). If using one of the Voice Remotes, like the original Roku 3 remote, their WiFi signal permits the Roku to be placed out of sight behind the TV or inside a closed cabinet if desired. If using the Players remote, for the IR signal to work, the Roku must be placed where the front of the Roku has an unobstructed line-of-sight from the location of the remote.
Note that there are numerous IR remotes from 3rd parties being sold as Roku replacements or as universal remotes that contain codes for Rokus. These should work as well but like the official Roku Players remote, they require line-of-sight to the front of the Roku. Make sure any 3rd party remote is for a Roku DEVICE and not a Roku TV.