Estimators for the scale of a Laplace distribution with location $\theta$

Let $X_1,X_2,\dots,X_n$ ($n\ge2$) be a random sample from a continuous distribution with the probability density function

$$f(x\mid\theta)=\frac1{2\theta}e^{-\frac{|x-\theta|}\theta},\quad x\in\mathbb R,$$

where $\theta(>0)$ is an unknown parameter. Let $U_n=\tfrac1n\sum_{i=1}^nX_i$, $V_n=\tfrac1n\sum_{i=1}^nX_i^2$, and $S_n^2=\tfrac1{n-1}\sum_{i=1}^n(X_i-U_n)^2$. Then, which of the following statements are true?