Chromium OS, like Chromium, is an open-source project initiated by Google. Its code is available for everyone to view, fork, modify, and build.
ChromeOS is a commercial product built on top of Chromium OS. It comes pre-installed on Chromebooks manufactured by Google-authorized vendors and is available for consumers to use.
FydeOS is a commercial product developed by Fyde Innovations based on the secondary development of Chromium OS. It adds many proprietary features on top of Chromium OS, providing a user experience similar to ChromeOS.