UMVUE of geometric parameter squared

Let $x_1=2, x_2=5$ and $x_3=4$ be the observed values of a random sample from a population having a probability mass function $$f(x; \theta)=\begin{cases} \theta(1-\theta)^x, & \text{if }x=0, 1, 2, \dots, \ 0, & \text{otherwise,} \end{cases}$$ where $\theta\in(0, 1)$ is an unknown parameter. If $\hat{\tau}$ is the uniformly minimum variance unbiased estimate of $\theta^2$, then $156\hat{\tau}$ equals __________