What was Malcom about to do when Macduff arrived in Act IV, scene 3?
I'll answer your questions about Shakespeare's Macbeth as best I can.
We don't get any information about what Malcolm was doing before Macduff arrives in England. We know Malcolm is there to ask for help to save Scotland from Macbeth, but the scene opens with both of them entering the stage together. Shakespeare apparently didn't consider this relevant.
Second, the effect of Malcolm's "discussion" or explanation of King Edward's healing powers is probably just to contrast him with Scotland's king, Macbeth. As Macbeth is evil, so Edward is righteous, at least in the play. Macbeth kills, Edward heals, etc. Malcolm's explanation also lets Macduff, as well as the audience, know that if/when Edward unites with Malcolm to dethrone Macbeth, goodness and righteousness will be battling evil. It gives a sense that good will prevail.
Finally, the characters you mention are ready to leave for Scotland and battle Macbeth. The only thing needed is for them to take their leave: "Macbeth is ripe for shaking."
Answer to 1: When Macduff arrived, Malcolm wants to test Macduff's loyalty. He also wants to bring out a clear idea about kingship from Macduff, and thus, wishes to see what sort of ruler the people want. The conversations nicely manifest the portrayal of an ideal kingship.
Answer to 2: There prevails the notion or belief in the Jacobean era that, kings have the divine power. Edward healed many people in his kingdom who were affected by evil spirits. Here also, Shakespeare shows a contradictory image between Macbeth the evil tyrant and Edward the divine healer. This gives a clear conception about ideal kingship.
Answer to 3: The three persons, being outraged against Macbeth, are getting ready to prepare for a battle against the bloody despot.