For a system of non-interacting bosons in a one-dimensional infinite potential well, the energy levels are given by:
\[
E_n = n^2 \times \frac{h^2 \pi^2}{2ma^2}
\]
For the third excited state, \( n = 4 \), so the energy is:
\[
E_4 = 4^2 \times \frac{h^2 \pi^2}{2ma^2} = 16 \times \frac{h^2 \pi^2}{2ma^2}
\]
Thus, \( x = 16 \).
The value of \( x \) is \( \boxed{16} \).