3) \(\frac{5}{21}+\frac{5}{28}\)

НОК(21, 28) = 84.

\[\frac{5}{21}+\frac{5}{28}=\frac{20}{84}+\frac{15}{84}=\frac{35}{84}=\frac{5}{12}.\]

Ответ: \(\frac{5}{12}\).