Well, it's always a good thing to start off with the definitions of efficiency, power, and power factor.
Let [tex]\eta[/tex] be the efficiency, then [tex]\eta = \frac{P_{out}}{P_{in}}[/tex]
Let P= power, then P=VI.
Therefore the average Power = [tex]VIcos(\theta) [/tex]
By definition of power factor, it is the ratio of average power/VI.
Thus, the definition of power factor = [tex]\frac{VIcos(\theta)}{VI}=cos(\theta)[/tex]
From here, use the Pythagorean theorem to solve for power factor of a three phase