3. In tetrahedron $ABCD$, the lengths of edges $AB$ and $CD$ are $a$ and $b$ respectively, and the distance between the midpoints of these two edges is $d$. Then the maximum volume of tetrahedron $ABCD$ is $\qquad$ (1994, Shanghai High School Mathematics Competition for Grade 10)
Answer: $\frac{1}{6} a b d$