3.8 7 21

$${\frac{3}{7}} \cdot {\frac{8}{21}} = {\frac{3 \cdot 8}{7 \cdot 21}} = {\frac{3 \cdot 8}{7 \cdot 3 \cdot 7}} = {\frac{8}{49}}$$

Ответ: $$\frac{8}{49}$$