3) \(\frac{3}{10}-\frac{4}{5}\); \(\frac{5}{7}-\frac{1}{2}\); \(\frac{3}{4}-\frac{5}{8}\).

1. \(\frac{4}{5}=\frac{8}{10}\).
   \(\frac{3}{10}-\frac{8}{10}=-\frac{5}{10}=-\frac{1}{2}\).
2. Общий знаменатель — 14:
   \(\frac{5}{7}=\frac{10}{14}\), \(\frac{1}{2}=\frac{7}{14}\).
   \(\frac{10}{14}-\frac{7}{14}=\frac{3}{14}\).
3. \(\frac{3}{4}=\frac{6}{8}\).
   \(\frac{6}{8}-\frac{5}{8}=\frac{1}{8}\).

Ответ: \(-\frac{1}{2};\ \frac{3}{14};\ \frac{1}{8}\).