Three crew members were aboard the Hail Mary before Ryland Grace became its sole survivor.
The Hail Mary is launched as humanity’s desperate mission to investigate why Tau Ceti remains unaffected by the astrophage crisis threatening the Sun and other stars. Grace awakens with severe amnesia and initially has to reconstruct both the mission and his identity from the ship’s records.
The crew consists of Grace and two other astronauts, who die during the journey after entering long-term coma. That leaves Grace alone aboard the spacecraft until he encounters Rocky, an Eridian astronaut whose own mission has also suffered catastrophic losses.
A common mix-up is to count only the two people who died, rather than the full original crew. “Three-person crew” includes Grace, making “three” the canonical answer.