The meaning and origin of the expression: Come the three corners of the world in arms

Come the three corners of the world in arms

What's the origin of the phrase 'Come the three corners of the world in arms'?

From Shakespeare's King John, 1595:

Now these her princes are come home again,
Come the three corners of the world in arms,
And we shall shock them. Nought shall make us rue,
If England to itself do rest but true.