Garland already faced two charges of first-degree murder for the deaths of Alvin and Kathy Liknes, but had been charged with second-degree murder last summer in their grandson's death.
A trial date is expected to be set on July 31.
During the preliminary hearing, which began last Tuesday, a number of witnesses were called, but their evidence is protected by a publication ban.
Garland sat in the prisoner's box every day, his legs shackled.
He displayed no emotion, but showed interest in the proceedings, often taking notes and leaning toward a screen used to display exhibits.
Preliminary trial well-attended
The courtroom was packed with police officers, victim assistance workers, and family and friends of the victims.
The first-degree charges indicate that investigators believe there was planning involved in the homicides.
Police confirmed on July 4 that there was blood found in the grandparents' home and that a violent incident had taken place.
Garland was taken in for questioning the next day, and officers began their extensive search of the Airdrie, Alta., acreage that he shared with his parents.
Bodies never found
Police also scoured several fields near Airdrie, sent divers into a pond and spent days at three local landfills.
More than 200 officers were involved in massive searches for Nathan and his grandparents. Police also followed up on about 1,000 tips from the public, but the three were never found.
Although Nathan and his grandparents were missing for two weeks before police laid murder charges, a court document shows investigators believe they died on June 30 — the day they were reported missing by Nathan's mother.
Garland has connections to the family, including past business dealings with Alvin Liknes.
Alvin Liknes's son was also in a common-law relationship with Garland's sister.