Monteith, who passed away on July 13 (13) in Vancouver, Canada from an accidental heroin and alcohol overdose, played Finn Hudson on the popular series.
Fox entertainment chairman Kevin Reilly, says, "The third episode will write Finn out of the show. That episode will deal directly with the incidents involved with Cory's passing and the drug abuse in particular."
The first two episodes, which are both Beatles' tributes, were shot when Monteith was alive and will feature a storyline revolving around the late star.
Reilly has confirmed reports suggesting the show's writers will take a three-week break after Monteith's swansong episode in order to "reset" and figure out how to proceed with the show without him.
Meanwhile, Glee creator Ryan Murphy will shoot public service announcements with the cast that will further address addiction issues.