The Amazon Fire HDX 8.9 has a problem with wirelessly mirroring its display with various devices like the Amazon Fire TV Stick, and unfortunately the Roku 3 is no exception.
The Fire HDX 8.9 sees the Roku 3 as one of the devices to mirror the display but consistently fails to connect. The tablet uses Miracast technology to support this wireless display mirroring but it only works with certain devices like the Belkin Miracast Video Adapter.