{ + \left( {1 - q} \right)p^2 \left( {1 - p} \right)\left( \begin{array}{l}
 f_1 \left( {n_1 + 1} \right)f_1 \left( {n_2 } \right)f_1 \left( {n_3 } \right) + \\ 
 + f_1 \left( {n_1 } \right)f_1 \left( {n_2 + 1} \right)f_1 \left( {n_3 } \right) + \\ 
 + f_1 \left( {n_1 } \right)f_1 \left( {n_2 } \right)f_1 \left( {n_3 + 1} \right) \\ 
 \end{array} \right)}