3. In tetrahedron $ABCD$, $AB=CD=a$, $BC=AD=b$, $CA=BD=c$. Find the cosine of the angle formed by the skew lines $AB$ and $CD$.
Construct a rectangular parallelepiped such that the face diagonals of the parallelepiped are the edges of the original tetrahedron. Thus, the angle formed by the skew lines $A B$ and $C D$ can be converted into the angle between two face diagonals. The required value is $\frac{\left|b^{2}-c^{2}\right|}{a^{2}}$.