jmc

We begin by finding the fractional forms of the decimals, $0.\overline{789}$, $0.\overline{456}$, and $0.\overline{123}$. Let $x=0.\overline{789}$, then $1000x=789.\overline{789}$ and $1000x-x=789.\overline{789}-0.789⟹999x-789$. Therefore, $0.\overline{789}=\frac{789}{999}$. We use the same method to find that $0.\overline{456}=\frac{456}{999}$ and $0.\overline{123}=\frac{123}{999}$. Next, we perform the indicated operations, knowing that $0.\overline{789}-0.\overline{456}-0.\overline{123}=\frac{789}{999}-\frac{456}{999}-\frac{123}{999}$. This equals $\frac{210}{999}$, which simplifies to $\boxed{\frac{70}{333}}$, when both the numerator and the denominator are divided by $3$.