The expression $\dfrac{x+y+z}{3}$ is the arithmetic mean (average) of $x$, $y$, and $z$.

Since $x < y < z$, the mean lies strictly between the smallest and largest values. However, we cannot determine whether the mean is greater than, equal to, or less than $y$ (the middle value) without knowing the exact values.

Example 1: $x=1, y=2, z=3$. Mean $= 2 = y$. Equal.

Example 2: $x=1, y=2, z=10$. Mean $= 4.33 > y$. Quantity A greater.

Example 3: $x=1, y=9, z=10$. Mean $= 6.67 < y$. Quantity B greater.

Since different cases give different results, the relationship cannot be determined from the given information.