When I try to open a Glyphs 3 file in Glyphs 2, it gives me a message that the file was created with a newer version of Glyphs and can’t be opened. If you are talking about the course by Juanjo López, “Diseño de una Tipografía digital”—loved that course—I think you could still do it with Glyphs 3. It’s been a while since I upgraded, so I don’t remember the specific changes, but I don’t think it’s different enough to matter.
I opened a Glyphs 3 file in FontLab 7 and then exported it as a Glyphs file. Glyphs 2 was able to open the exported file. I’m not sure if anything was lost in the conversion process, but it seemed to work.