This is LaTeX code:

[latex]
 
+ \left( {1 - q} \right)p\left( {1 - p} \right)^2 \left( \begin{array}{l}
 f_1 \left( {n_1 } \right)f_2 \left( {n_2 ,n_3 } \right) + \\ 
 + f_1 \left( {n_2 } \right)f_2 \left( {n_1 ,n_3 } \right) + \\ 
 + f_1 \left( {n_3 } \right)f_2 \left( {n_1 ,n_2 } \right) \\ 
 \end{array} \right) +
 
[/latex]