One-Bullet Dilemma

Three prisoners. One bullet each. Last one alive goes free. What would you do?
In the near future, against the backdrop of a recent alien invasion, Mark, Willa, and Rasheed wake up in adjoining prison cells. Each has a gun with a single bullet.
These invaders are masters at psychological manipulation. Human divisions are the aliens' greatest weapon. Can Mark, Willa, and Rasheed overcome those differences, or will millions die with a single trigger pull?