3) \frac{3}{14} x - \frac{4}{21} x + \frac{7}{28}x;

3) $$\frac{3}{14}$$ x - $$\frac{4}{21}$$ x + $$\frac{7}{28}$$x = x($$\frac{3}{14}$$ - $$\frac{4}{21}$$ + $$\frac{7}{28}$$) = x($$\frac{3 \cdot 6}{14 \cdot 6}$$ - $$\frac{4 \cdot 4}{21 \cdot 4}$$ + $$\frac{7 \cdot 3}{28 \cdot 3}$$) = x($$\frac{18}{84}$$ - $$\frac{16}{84}$$ + $$\frac{21}{84}$$) = x$$\cdot$$\frac{18 - 16 + 21}{84}$$ = x$$\cdot$$\frac{23}{84}$$ = $$\frac{23}{84}$$x

Ответ: $$\frac{23}{84}$$x