Four most important events of act 3 are the following:
1) Macbeth hosting his coronation banquet;
2) Macbeth talking to two murderers and finalizing the killing of Banquo and his son, Fleance;
3) The murderers killing Banquo with Fleance escaping;
4) The appearance and re-appearance of the ghost of Banquo at the banquet.
The second part of your question sounds a bit confusing to me. Still I am trying to make some comments on each of the four events already mentioned:
1) Macbeth specially invites Banquo to be present at the banquet as the chief guest and the latter promises that he would not fail. Macbeth then speaks his mind in a soliloquy that reveals as to why he wants to get rid of Banquo. The banquet later exposes Macbeth's villainy in the assembly of all Scottish nobles.
2) As we see how Macbeth finds out two men to antagonize them against Banquo, we understand that Macbeth can now work out his evil designs by himself and in a very cleverly manipulating manner.
3) As the rain of blows on Banquo's head dispatch him to death, we see how Macbeth enhances his own doom. Banquo shall have to keep his promise of attending the banquet in the form of a ghost born of Macbeth's own terror-stricken mind, since Fleance has escaped.
4) Banquo's ghost appears and disappears twice in the banquet scene. Dead Banquo thus proves to be more potent a threat to Macbeth than the living Banquo. The ghost forces Macbeth to betray his crime and prepares his way to doom as Macbeth resolves to meet the witches to know 'by the worst means, the worst'.
Hope you will find all this reasonably helpful.