We present three-dimensional simulations of optical-parametric chirped-pulse amplification stages for a few-cycle petawatt-class laser. The simulations take into account the effects of depletion, diffraction, walk-off, quantum noise, and the nonlinear refractive index ( $n_2$ ). In the absence of $n_2$ effects, we show these stages can generate $3.67\mathrm{J}$ pulses supporting $4\mathrm{fs}$ transform-limited pulse durations. Adding the nonlinear refractive index to the simulation, the energy output is reduced by $\sim 11\%$ and the bandwidth narrows by $\sim 129\mathrm{nm}$ , increasing the Fourier limit by $\sim 17.5\%$ .