\[\boxed{\mathbf{3}.}\]
\[1)\ \left( - \frac{2}{3}a^{2}b^{6}c \right)^{3} \cdot 9a^{5}bc^{2} =\]
\[= - \frac{8}{27}a^{6}b^{18}c^{3} \cdot 9a^{5}bc^{2} =\]
\[= - \frac{8}{3}a^{11}b^{19}c^{5} =\]
\[= - 2\frac{2}{3}a^{11}b^{19}c^{5}\]
\[2)\ 12x^{2}yz^{7} \cdot \left( 0,5x^{3}y^{8} \right)^{2} =\]
\[= 12x^{2}yz^{7} \cdot 0,25x^{6}y^{16} =\]
\[= 3x^{8}y^{17}z^{7}\]
\[3)\ \left( 36m^{8}n^{2}k \right)\ :\left( 12m^{2}n \right) =\]
\[= 3m^{6}\text{nk}\]
\[4)\ \left( - \frac{5}{9}a^{9}b^{8}c^{7} \right)\ :\left( 5a^{3}b^{3}c \right) =\]
\[= - \frac{1}{9}a^{6}b^{5}c^{6}\]