A and B can do a piece of work in 18 days;

Time taken by $(\mathrm{A}+\mathrm{B})$ to do the work $=18$ days

Time taken by $(\mathrm{B}+\mathrm{C})$ to do the work $=24$ days

Time taken by $(\mathrm{A}+\mathrm{C})$ to do the work $=36$ days

Now,

Work done by $(\mathrm{A}+\mathrm{B})=\frac{1}{18}$

Work done by $(\mathrm{B}+\mathrm{C})=\frac{1}{24}$

Work done by $(\mathrm{A}+\mathrm{C})=\frac{1}{36}$

$\therefore$ Work done together $=(\mathrm{A}+\mathrm{B})+(\mathrm{B}+\mathrm{C})+(\mathrm{A}+\mathrm{C})$

$=\frac{1}{18}+\frac{1}{24}+\frac{1}{36}$

$=\frac{4+3+2}{72}=\frac{9}{72}$

$=\frac{1}{8}$

$\therefore$ Work done together $=2(\mathrm{~A}+\mathrm{B}+\mathrm{C})=\frac{1}{8}$

$\therefore$ Work done by $(\mathrm{A}+\mathrm{B}+\mathrm{C})=\frac{1}{16}$

Thus, together they can finish the work in 16 days.