解：$(1)$∵$\frac {x}{6ab²}$和$\frac {y}{9a²bc}$的最简公分母是$18a²b²c$，∴$\frac x{6ab²}=\frac {3acx}{18a²b²c}$，$\frac {y}{9a²bc}=\frac {2by}{18a²b²c}$
$(2)$∵$\frac {2}{4-9m²}$，$\frac {3}{9m²-12m+4}$的最简公分母是$(2+3m) (2-3m)²$
∴$\frac {2}{4-9m²} =\frac {2(2-3m)}{(2+3m)(2-3m)²} $，$ \frac {3}{9m²-12m+4} =\frac {3(2+3m)}{(2+3m)(2-3m)²} $
$(3)$∵$\frac {1}{x²-5x}$，$\frac {2}{x²-25}$，$\frac {3}{x²+5x}$的最简公分母是$ x (x + 5) (x - 5)$
∴$\frac {1}{x²-5x} =\frac {x+5}{x(x+5)(x-5)} $，$\frac {2}{x²-25} = \frac {2}{(x+5)(x-5)}$，$\frac {3}{x²+5x} =\frac {3(x-5)}{x(x+5)(x-5)}$
