3) 9 8/21 + 4 11/14 ;

3) $$9 \frac{8}{21} + 4 \frac{11}{14} = (9 + 4) + (\frac{8}{21} + \frac{11}{14}) = 13 + (\frac{8 \cdot 2}{21 \cdot 2} + \frac{11 \cdot 3}{14 \cdot 3}) = 13 + (\frac{16}{42} + \frac{33}{42}) = 13 + \frac{49}{42} = 13 + \frac{7}{6} = 13 + 1 \frac{1}{6} = 14 \frac{1}{6}$$

Ответ: $$14 \frac{1}{6}$$