3. $$3\frac{9}{10}-\left(2-1\frac{11}{35}\right):\frac{9}{49}+\frac{1}{2}$$

Решим пример по действиям:

1. $$2 - 1\frac{11}{35} = 2 - \frac{46}{35} = \frac{70}{35} - \frac{46}{35} = \frac{24}{35}$$
2. $$\frac{24}{35} : \frac{9}{49} = \frac{24}{35} \cdot \frac{49}{9} = \frac{8 \cdot 3}{5 \cdot 7} \cdot \frac{7 \cdot 7}{3 \cdot 3} = \frac{8 \cdot 7}{5 \cdot 3} = \frac{56}{15} = 3\frac{11}{15}$$
3. $$3\frac{9}{10} - 3\frac{11}{15} = \frac{39}{10} - \frac{56}{15} = \frac{39 \cdot 3}{10 \cdot 3} - \frac{56 \cdot 2}{15 \cdot 2} = \frac{117}{30} - \frac{112}{30} = \frac{5}{30} = \frac{1}{6}$$
4. $$\frac{1}{6} + \frac{1}{2} = \frac{1}{6} + \frac{1 \cdot 3}{2 \cdot 3} = \frac{1}{6} + \frac{3}{6} = \frac{4}{6} = \frac{2}{3}$$

Ответ: $$\frac{2}{3}$$