The effect of counter‐rotating streamwise steady vortices on three‐dimensional Tollmien–Schlichting (TS) waves in a growing boundary layer on a concavely curved surface has been studied by using the method of multiple scales. The nonlinear evolution of the amplitudes with streamwise position has been obtained analytically. It is shown that the TS waves are amplified by the presence of streamwise vortices. The effect of combination resonance between TS waves and Goertler vortices is much more pronounced than the effect of parametric resonance between the TS waves and the periodic basic flow. The amplification rates increase with Goertler number and spanwise wavenumber.