Heat transport in 3He above its critical temperature Tc was studied along the critical isochore in a flat Rayleigh-Bénard cell (height h=1 mm, diameter D=57 mm). The range of the reduced temperature epsilon was 5 x 10(-4)< or = epsilon < or =2 x 10(-1). The temperature difference deltaT(t) across the fluid layer as a function of the time t was measured for different values of the heat current q until steady state was reached. The crossover was observed from the regime dominated by the Rayleigh criterion for the convection onset to that controlled by the adiabatic temperature gradient (ATG), or "Schwarzschild criterion," in good quantitative agreement with predictions. The slope of the convective heat current versus the reduced Rayleigh number was found to be independent of compressibility and the same as for still less compressible fluids. Plots of Nu versus Ra, both corrected for the ATG effect, are presented for early-stage convective turbulence (1 x 10(5)