3. $$rac{2a(b+c)}{4a^3(b+c)(b-c)}$$

$$\frac{2a(b+c)}{4a^3(b+c)(b-c)} = \frac{2}{4} \cdot \frac{a}{a^3} \cdot \frac{(b+c)}{(b+c)} \cdot \frac{1}{(b-c)} = \frac{1}{2} \cdot \frac{1}{a^2} \cdot 1 \cdot \frac{1}{(b-c)} = \frac{1}{2a^2(b-c)}$$

Ответ: $$\frac{1}{2a^2(b-c)}$$