  The probability distribution of the total transmission is studied for waves
multiple scattered from a random, static configuration of scatterers. A
theoretical study of the second and third cumulant of this distribution is
presented. Within a diagrammatic approach a theory is developed which relates
the third cumulant normalized to the average, $\langle \langle T_a^3 \rangle
\rangle$, to the normalized second cumulant $\langle \langle T_a^2 \rangle
\rangle$. For a broad Gaussian beam profile it is found that $\langle \langle
T_a^3 \rangle \rangle= \frac{16}{5} \langle \langle T_a^2 \rangle \rangle^2 $.
This is in good agreement with data of optical experiments.
