Linear unbiased estimator with smallest variance

Let $X_1,X_2,X_3$ be a random sample from a distribution with the probability density function

$$f(x|\theta)=\begin{cases}\dfrac1\theta e^{-\frac x\theta}, & x>0,\ 0, & \text{otherwise,}\end{cases}\quad\theta>0.$$

Which of the following estimators of $\theta$ has the smallest variance for all $\theta>0$?