Depression in freezing point ($\Delta T_f$) is a colligative property proportional to the number of solute particles in the solution. It is calculated using the formula $\Delta T_f = i \times K_f \times m$, where $i$ is the van't Hoff factor.

Since the solutions are dilute (0.01 M) and the solvent is the same, we assume molality is proportional to molarity and $K_f$ is constant. Thus, the ratio of $\Delta T_f$ depends only on the van't Hoff factor ($i$):

1. Urea: A non-electrolyte, does not dissociate. $i = 1$.
2. Sodium Chloride ($NaCl$): A strong electrolyte, dissociates as $NaCl \rightarrow Na^+ + Cl^-$. It gives 2 ions, so $i = 2$.
3. Sodium Sulphate ($Na_2SO_4$): A strong electrolyte, dissociates as $Na_2SO_4 \rightarrow 2Na^+ + SO_4^{2-}$. It gives 3 ions, so $i = 3$.

The ratio of depression in freezing point is $1 : 2 : 3$.