The triple differential cross sections for electron impact ionization of hydrogen at incident energies of 54.4, 150 and 250eV in the coplanar asymmetric geometry are calculated by use of BBK model and modified BBK model. The present results are compared with the results of first order approximation and experimental measurements. The non-first-order effects of modified BBK model are analysed and discussed. It is found that screening of three bodies in the final state conveys rich information of the non-first-order effects.