3. \(\frac{7}{15}\cdot\frac{5}{21}\)

Сократим: \(7\) и \(21\) на \(7\), \(5\) и \(15\) на \(5\).

\(\frac{7}{15}\cdot\frac{5}{21}=\frac{1}{3}\cdot\frac{1}{3}=\frac{1}{9}\).

Ответ: \(\frac{1}{9}\).