What three questions does Marcellus raise in the first act ,scene one?

Marcellus asks, "Who hath relieved you?" "What, has this
thing appear'd again to-night?" and "Is it not like the King?"
(I.i).

Marcellus is a guard of the court who arrives
outside the castle with Horatio, a friend of Prince Hamlet. They are on duty and discuss
a ghost that has appeared to them. Since Horatio is a scholar and only believes what he
is able to see, he has come to see the ghost himself. They are not sure if the ghost is
a warning or an evil spirit there to pull them into Hell. Since Horatio knows Latin, it
is believed that if the ghost is evil, he will be able to perform an
exorcism.

When the ghost appears, it looks very much like
the late King of Denmark, and it is wearing the King's armor. The ghost does not speak.
At the end of the scene Horatio decides that he must go tell Prince Hamlet of the
sighting. This act foreshadows Horatio's loyalty to Prince
Hamlet.