Let the total number of oranges be $x$. The first boy receives $\frac{1}{3}x$, leaving $\frac{2}{3}x$. The second boy then takes $\frac{2}{3}$ of the remainder, which is $\frac{2}{3} \times \frac{2}{3}x = \frac{4}{9}x$, leaving $\frac{1}{3}x$ for the third boy, who receives 12 oranges, leading to the equation $\frac{1}{3}x = 12$ and solving gives $x = 36$, thus confirming the total shared is 54 oranges.