The electric field parallel to the magnetic field in nonlinear magnetosonic waves in three component plasmas (two-ion-species plasma and electron-positron-ion plasma) is theoretically studied based on a three-fluid model. In a two-ion-species plasma, a magnetosonic mode has two branches, high-frequency mode and low-frequency mode. The parallel electric field E-parallel to and its integral along the magnetic field, F = -integral E(parallel to)ds, in the two modes propagating quasiperpendicular to the magnetic field are derived as functions of the wave amplitude epsilon and the density ratio and cyclotron frequency ratio of the two ion species. The theory shows that the magnitude of F in the high-frequency-mode pulse is much greater than that in the low-frequency-mode pulse. Theoretical expressions for E-parallel to and F in nonlinear magnetosonic pulses in an electron-positron-ion plasma are also obtained under the assumption that the wave amplitudes are in the range of (m(e)/m(i))(1/2) < epsilon < 1, where m(e)/m(i) is the electron to ion mass ratio. Published by AIP Publishing.