3 Actors in Greek Drama

I find it to be extraordinary (if true), but in our book club discussion of Euripedes’ Hippolytus and Racine’s Phaedre (both extraordinary works, btw), a drama teacher mentioned that none of the Greek dramas used more than 3 actors (and an unspecified number of chorus members). For all additional characters, actors donned masks or costumes to differentiate themselves.