Let \( u(x, t) \) be the solution of the wave equation
\[
\frac{\partial^2 u}{\partial t^2} = \frac{\partial^2 u}{\partial x^2}, \quad 0<x<\pi, \, t>0,
\]
with the initial conditions
\[
u(x, 0) = \sin x + \sin 2x + \sin 3x, \quad \frac{\partial u}{\partial t}(x, 0) = 0, \quad 0<x<\pi,
\]
and the boundary conditions
\[
u(0, t) = u(\pi, t) = 0, \quad t \geq 0.
\]
Then, the value of \( u \left( \frac{\pi}{2}, \pi \right) \) is