Why is the entropy of container A the greatest?
An accurate summary of the relative molar heat capacities of the different species is given in another answer and recapitulated in the following table together with the total heat capacity for each gas:

$$\begin{array}{|case|Cv|} \hline \textrm{molecule} &amp; C_{vm}/R &amp; NC_{vm}/R \\ \hline  \mathrm{A} &amp; 3/2 &amp; 1.5 \\ \mathrm{B} &amp; 5/2 &amp; 1.25  \\\mathrm{C} &amp; 7/2 &amp; 1.75 \\ \hline \end{array}$$

where $N$ is the total amount of gas, which based on the problem described in the OP is either $\pu{1 mol}$ of atoms ($\mathrm{A}$) or $\pu{\frac{1}{2} mol}$ of homonuclear diatomic molecules ($\mathrm{B}$ and $\mathrm{C}$).

Since the entropy change for the isochoric temperature change is $\Delta S = C_v \log\left( \frac{T_2}{T_1} \right)$, it follows that $\Delta S_C &gt; \Delta S_A &gt; \Delta S_B$.