Evaporation of sessile droplets is an important fundamental problem. The present three-dimensional numerical model considers various effects including vapor diffusion, buoyancy-driven flow, and evaporative cooling, with conjugate heat and mass transfer solved throughout the computational domain. Evaporation of a sessile water droplet with an initial volume of 3 μL is investigated on heated horizontal superhydrophobic substrates. A non-axis-symmetric rolling flow is presented inside the droplet rather than the axis-symmetric recirculation flow predicted by a two-dimensional axisymmetric model. This is because the axis-symmetric flow is not stable on heated superhydrophobic substrate and sensitive to any perturbations. The evaporative cooling along the droplet interface is observed, but the coolest point appears on the one side of the droplet instead of the droplet top owing to the rolling flow inside the droplet. Influence of relative humidity is also discussed and it indicates a stronger impact on substrates with relatively lower temperature. The present numerical study reveals the important underlying transport characteristics, which provides new insights into evaporation of water droplets resting on heated horizontal superhydrophobic substrates.