Yes. R3 is written in C and Rebol. Red is so far using Rebol2 to be compiled and Red for mezzanine code.
Main difference is that there is a GUI available in Red. That is not available in R3 yet (at least not in my version). There was a GUI in the Saphirion fork, but it is abandoned for years and I didn't wanted to use it. I would like to provide GUI in the future. Probably as a native extension.
The code is almost same.. although there may be differencies in some function refinements or other details.
Even when Red is compiled to the assembly, it is still slower than interpreted R3 code, because of lack of optimizations. As it depends on its own compiler is also the reason, why there still isn't support fot 64bit arch. To write own compiler is a big task indeed.
And then R3 and Red differs in the number of developers. There may be 3 or 4 people (2 active) working on Red while R3 is worked just by me so far.