Linear Models — BLUE of theta1 from combined sums

Let $X_i,Y_i,Z_i$ where $i=1,2,3$ be nine independent observations with common variance $\sigma^2$, and $E(X_i)=\theta_1$, $E(Y_i)=\theta_2$ and $E(Z_i)=\theta_1-\theta_2$; $i=1,2,3$. Let $X=\sum_{i=1}^3 X_i$, $Y=\sum_{i=1}^3 Y_i$ and $Z=\sum_{i=1}^3 Z_i$. What is the BLUE of $\theta_1$ equal to?