The average thermal energy for a mono-atomic gas is: ($k_B$ is Boltzmann constant and $T$, absolute temperature)

According to the law of equipartition of energy, each degree of freedom contributes $\frac{1}{2}{k}_{B}T$ to the average energy. A mono-atomic gas has 3 degrees of freedom (all translational).
$$\text{Average energy}=3\times \frac{1}{2}{k}_{B}T=\frac{3}{2}{k}_{B}T$$