Red is not compatible with Rebol3, due in part to the fact that development on Red started before Rebol3 was open-sourced. The binaries for Rebol3 at the time were too unstable, so the older and much more solid Rebol2 was chosen. There is a major undertaking underway currently though to refactor and fix large portions of Rebol3's core code. Which will then be rolled into the community edition and also hopefully the corporate sponsored Atronix fork. Also there is some talk going around about packaging and integrating TinyCC with Rebol3. IMO both projects show alot of promise, and have many progressive ideas.
R3 is open source (Apache License), but Rebol 2 is proprietary.