3. (sin²a+tga-sin² a) ctga;

3. $$(\sin^2 \alpha + tg^2 \alpha \cdot \sin^2 \alpha) \cdot ctg \alpha = \sin^2 \alpha (1 + tg^2 \alpha) \cdot ctg \alpha = \sin^2 \alpha \cdot \frac{1}{\cos^2 \alpha} \cdot \frac{\cos \alpha}{\sin \alpha} = \frac{\sin \alpha}{\cos \alpha} = tg \alpha$$

Ответ: $$tg \alpha$$