Solution:

The answer is (D). The treasure is buried at the circumcenter $O$ of the triangle at whose vertices Alice, Berto, and Carlo are located. Since this triangle is right-angled, $O$ is the midpoint of the hypotenuse. Letting $x$ be its length expressed in meters, we have $x+\frac{x}{2}+\frac{x}{2} \sqrt{3}=6$, from which $\frac{3+\sqrt{3}}{2} x=6$ and $x=2(3-\sqrt{3})$. Therefore the distance of the treasure from each of them is $3-\sqrt{3}$.