1. Scene 1
"for the wise men know well enough what monsters you make of them"
The "monsters" are a symbol in the play for the evil nature of the people in the play. In that a monster is a symbol of evil, and can represent parallely the true nature of such characters as King Claudius.
2. Scene 2
"It out-Herods Herod."
This provides an allusion to King Herod in the holy bible. In the scene, Hamlet is comparing this tragic event of the play not being held in a high honor, and how awful a punishment would be received if honor was not recieved. This "punishment" would out Herod Herod, in that it alludes to King Herod and the awful things that he proved himself capable of while he was King. While King Herod was in control, he was very ruthless and power hungry in that he would do anything to ensure his reign as king for as long as he lived. While he was King, Jesus Christ was born as "King of the Jews," and when King Herod heard of this news, he was filled with jealousy and began his search for Jesus by killing all the newborn boys in Israel. To say that Hamlet's "punishment" for being unhonorable is greater than the capabilities of King Herod's punishments would mean that the punishment is one of great measure.
3. Scene 3
"And my imaginations are as foul as Vulcan's stithy."
This provides an allusion to the Roman God Vulcan, who is the God of fire and metalworkings. Vulcan is connected to the destructive powers of fire and is supposedly used against enemies. In this, Vulcan's "stithy" is his blacksmith tools. Hamlet is making a comparison of his own intended behavior to the Godly, most powerful, toughest, and most dangerous blacksmith of all.