#1. After cutting Keyes down and on your way to get the flight recorder, you'll encounter multiple xenos for the first time once you enter the next large room. Before moving forth, head to the right side from the entrance and check the corner near the railing for the first dog tag.
#2. While navigating the gravity well, you'll come across a room with an audio log, take the hallway past that and before exiting to the catwalk, you'll find the second dogtag on the shelf beside the door.
#3. After Oneal successfully cuts open the sealed door to the cargo bay control room, don't activate the purge lever yet. Check crate leading to the control room's second door at the back to find this dog tag.