What three messages does Macbeth receive from the three apparitions?

OK, so we're looking here at the apparitions scene: Act 4, Scene 1. Let's go one by one.

The first apparition is an "Armed Head". He says:

Macbeth! Macbeth! Macbeth! Beware Macduff;
Beware the Thane of Fife. Dismiss me. Enough.

Macduff, though no-one knows this yet, will be the one to eventually kill Macbeth. So it's good advice - and it's simple: Beware of Macduff (who is also the Thane of Fife).

THe second apparition is a bloody child.

Be bloody, bold, and resolute; laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth.

Macbeth is to be bloody (i.e. shed lots of blood), brave and resolved. No-one born of a woman can harm him. The irony, which we don't discover until the last scenes of the play, is that Macduff was not "born", in the normal way, but born by section, cut out of his mother's womb.

The third apparition is a baby, crowned, with a branch in his hand. He says to Macbeth:

Be lion-mettled, proud, and take no care
Who chafes, who frets, or where conspirers are:
Macbeth shall never vanquish’d be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him.

So Macbeth is to be as brave as a lion, and to ignore everyone else. Macbeth will never be overcome ("vanquished") until the nearby wood, Birnam Wood, moves to Dunsinane Hill.