The main difference among the three characters is the ability or lack of ability to act decisively. Throughout the first four and half acts of the play, Hamlet wrestles with whether or not he should avenge his father's death and then how he should proceed with the revenge. As soon as Laertes finds out his father was killed, he immediately demands revenge. Fortinbras goes to the extent to lie to his uncle in order to exact his revenge against Denmark for his father's death. Both Laertes and Fortinbras act without delay and with determination while Hamlet struggles in a sea of indecision.
Fortinbras is decisive and warrior-like whereas Hamlet vacillates and is much more peace-loving. Fortibras is willing to conquer a relatively useless piece of land simply to enlarge the conquests of his kingdom: (Truly to speak, and with no addition,
We go to gain a little patch of ground / That hath in it no profit but the name. To pay five ducats, five, I would not farm it;
Nor will it yield to Norway or the Pole / A ranker rate, should it be sold in fee (4.4.119-125).
Laertes is easily led while Hamlet needs to be sure of everything before he makes a move, yet he feels stymied by doubt, as evidence in his famous "To be or not to be" soliloquy (Act 3.1). But Laeretes is all to ready to believe the worst, and be sucked in by Claudius' deceit, becoming a pawn to his deadly games.
Hamlet_ He is a very scholarly and unscrupulous person.He does not follow blindly but he examines and collects information.He belives in religious rewards and knows what is bad and what is good.It is his good and religious spirit for that delay occurs that becomes the cause of his tragedy.
Fortinbras_ He is very different person thinking of enlarging the
area of his country .For little piece of land he is ready to get killed
the army. He gives importance to land and conquest,where as Hamlet to humanity and justice.
Laertes_ He is very different from the both.He seems credulous
and easily moveable.He is ready to revenge from Hamlet without getting any proof.He is always under the shadow of Claudius conspiracy.It means he does not have distinguishing power.That`swhy he is creating trouble for Hamlet.