3. \(\sqrt{8c}-\sqrt{50c}+\sqrt{18c}\)

При \(c\geqslant0\):

\(\sqrt{8c}=2\sqrt{2c}\), \(\sqrt{50c}=5\sqrt{2c}\), \(\sqrt{18c}=3\sqrt{2c}\).

Тогда:

\(2\sqrt{2c}-5\sqrt{2c}+3\sqrt{2c}=0\).

Ответ: \(0\)