What is the significance of Act 1, scenes 3-6, of Macbeth?
The opening scenes of many plays tend to serve the same purpose--to introduce characters, plot, setting, initial conflict and mood. This is no different with William Shakespeare's Macbeth. While the first two scenes are not being referred to in the question, the opening scene of the play sets the ominous and dark mood of the scenes to follow (developed through the use of the witches casting spells).
The scenes in question, three (3) through six (6), are significant given they set up Macbeth's path throughout the play. In scene three, Macbeth comes to find out that he will obtain the crown one day. This hope pushes Macbeth's already large ambitious nature, insuring that Macbeth will do whatever it takes to obtain the crown.
Scene four sets into motion the conflict which will arise between Macbeth and Malcolm (given Malcolm has been named to take the throne after his father's death).
Scene five is important because it shows Lady Macbeth to be as cold-hearted and as ambitious as her husband. Readers learn that Lady Macbeth will stop at nothing to have her husband king (even changing her own gender--given she does not believe her husband is "man enough" to do what needs to be done).
In the end, the scenes in question are significant given they establish better (deeper) understanding of the characters (especially Lady Macbeth and Macbeth), set up conflict, and show where the conflict stems from (the prophecies).