Salior/The Messenger : The Salior brings Horatio a letter from Hamlet exclaiming that he has been taken prisoner by Pirates, yeah... that kind of came out of no where but moving on, and that he has very important news for Claudius. The messenger then brings Hamlet's letter to Claudius in which Hamlet proceeds to tell him that he is coming back to Denmark. So basically The Sailor's and The Messenger's only purpose in this Act is to provide a twist.
Fortinbras: Plays a minor roll in this Act. He basically just tells Hamlet that Fortinbras asks permission to move his troops across Denmark.
Captain: The Captain's, just like Fortinbras, main purpose is to enlighten Hamlet with the news that the nephew of the old King of Norway is directing troops to poland to fight over some worthless land.