Sita can finish typing a 100 page document in 9 hours.

Time taken by Sita to do the work $=9$ hours

Time taken by Mita to do the work $=6$ hours

Time taken by Rita to do the work $=12$ hours

Now,

Work done by Sita $=\frac{1}{9}$

Work done by Mita $=\frac{1}{6}$

Work done by Rita $=\frac{1}{12}$

$\therefore$ Work done by them together $=\frac{1}{9}+\frac{1}{6}+\frac{1}{12}$

$=\frac{4+6+3}{36}=\frac{13}{36}$

Thus, together they can do the work in $\frac{36}{13}$ hours.