f_3 \left( {n_1 ,n_2 ,n_3 } \right) = p_3 \left[ {q\left( {1 - p} \right)^3 + qp\left( {1 - p} \right)^2 \left( {f_1 \left( {n_1 } \right) + f_1 \left( {n_2 } \right) + f_1 \left( {n_3 } \right)} \right) + } \right.