How do Stephano, Trinculo and Caliban respond to the magic of the island in Act II, scene 3 and Act IV, scene 1?
Perhaps you mean Act III, scene 11 in which Caliban persuades Stephano and Trinculo to kill Prospero. Ariel is present, though invisible, listening to the three as they plan their cruel plot. After singing a raucous tune, the three are surprised to hear the same tune played on a tabor and pipe. Of course, Ariel played the tune to charm the fools and lead them to a foul pool of water. Stephano and Trinculo are afraid of the music at first and believe it comes from a demon spirit. However, Caliban assures them that the music is harmless just as are all the often heard noises of delight that fill the island.
I'm afraid my version of the script doesn't have a third scene for Act 2. For Act 4, scene 1, at first Stephano and Trinculo don't really realize magic has been done on them. They blame Caliban. Then, they are awestruck by Prospero's wealth, still without realizing any of it is magical. They are taken in by the magic, in part precisely because they don’t realize it is happening. Caliban in this scene is wiser than they are; he's afraid. Their final response in this scene is to run away, herded by the spirits Prospero's called. They are therefore afraid and helpless.