A nonlinear interaction in three-dimensional transition to turbulence is presented. The author focuses on the transition through subharmonic resonance in the upper branch scaling regime in an accelerating boundary layer flow. The author’s results exhibit that the interaction between planar and oblique modes produces a jump across the critical level. Quadratic interactions of oblique modes generate a spanwise-dependent mean flow, which becomes unbounded at the edge of the critical layer. Then a diffusion layer is introduced, where the spatial disturbances are balanced by viscous effects. Due to the cubic interaction between obliques modes, the mean flow produces another jump at leading order, which is then extended to the unequal-amplitude case. The numerical results show that when oblique modes start with a relatively small amplitude, the growth is fast; when the amplitude becomes quite large, the cubic interactions of the oblique modes become effective, inhibiting the growth and causing oscillation of the amplitude; when the initial amplitude of oblique modes is sufficiently large, the parametric resonance can be completely bypassed with the results that the amplitude grows exponentially. The effects of both a phase distance and wavenumber detuning are also studied, and both factors significantly influence the development of the waves.