This achievement is unlocked in the chapter titled "Hide and Seek" (roughly the 10th chapter, depending on the order you progress the game in).
Part way through the chapter, you'll have a flashback to Lucas as a child. After a cutscene, when you start to move, there will be some visions with a QTE which you have to win to unlock this achievement.
After this you need to work your way to the hangar and into it through the hole in the back wall. (I'm skipping those instructions since they are simply part of completing this chapter) Once inside, a timer will start. In order to get the achievement, you have to find all three boys before the timer runs out and there isn't a lot of time to spare. They are sort of hard to find, even if you know exactly where to look.
1. The hardest one to find, he's inside a crate on the lower floor in the corner at the far end of a narrow path between crates, more or less straight down from where you start (although you have to go down and to the right and then back to get to him), and even then you can't really see him but just have to face exactly the right direction to find him.
2. In the back of part of an airplane on the ground near the base of the stairs. You need to circle to the back of the airplane part and face it to find him. You also have to convince him to leave (mention his mother).
3. The easiest one to find, in the upper platform toward the far end of the stairs, behind a wooden palette.
If you find all three before the timer runs out, the achievement will unlock. If it looks like you're going to run out of time, you can quit and continue at the last save point which was when the timer started upon entering the hangar.