These three characters make up a three-person character foil. A foil is a character set up in contrast to another one who has similar characteristics. All three of these young men, in Act IV, are approximately the same age, have all lost a father, and all have a legitimate claim for revenge.
However, all three differ in how they approach this revenge duty. Hamlet, as we have seen all throughout the play, has taken a type of avoidance tactic. He is circumventing his duty, refusing to act directly. Conversely, Laertes is direct to a fault; he instantly attacks the king, blaming him for his father's death. He doesn't stop to think logically at all. In this way, he is the direct opposite of Hamlet. Finally, Fortinbras seeks his revenge in a calculated, logically planned manner. He devises a plan to sweep across Denmark and take control by surprise; he is driven to a degree that literally lacks emotion.
These three men all take the same set of circumstances and create their own, very different, outcomes.