We solve the elasto-dynamic problem for a 3-D rupture, spontaneously propagating on a fault, obeying rate- and state-dependent friction. We explore, through numerical simulations with physically realistic constitutive parameters, the effects on dynamic traction evolution of the flash heating of microscopic asperity contacts. Our results demonstrate that the inclusion of flash heating tends to increase the degree of instability of a homogeneous fault: the supershear rupture regime is favored, significantly larger stress drops are realized and weakening distance and fracture energy increase. We show that the key parameter which controls the temperature evolution and the activation of the flash heating is the slipping zone width, 2w. We found that for localized shear the rupture exhibits a pulse-like behavior. On the contrary, for large slipping zones, the rupture develops as a sustained crack. Finally, we show that flash heating enhances the onset of melting.