1. Polonius: He stabs Polonius through the arras (tapestry) thinking it is Claudius.
2. Claudius: He poisons Claudius with a poisoned cup during the fencing match.
3. Laertes: He is also killed during the fencing match, though not intentionally by Hamlet. Hamlet was given the poisoned sword, unknowingly.
It's important to remember that Hamlet also contributes to the deaths of Ophelia and Rosencrantz and Guildenstern, though he doesn't directly kill them.