At the main menu click Freeplay and then do Mira HQ.
As soon as the game loads go to the Laptop, open the Laptop and click on Imposter.
The crewmates are in storage, balcony, admin, greenhouse, laboratory, and locker room
From here I thought the best way was to start in storage and kill Blue. After killing Blue walk South to the balcony and kill Pink. After killing Pink use the vent to the right in the balcony and go up twice to admin and kill Black. After killing Black report the body for your three kills.
You can also sabotage the lights in the office for an extra achievement.
You do need to load into an online game once and then back out before going to Freeplay.