On 2017 September 6, the solar active region 12673 produced an X9. 3 flare, regarded to be the largest to have occurred in solar cycle 24. In this work we have performed a magnetohydrodynamic (MHD) simulation in order to reveal the three-dimensional (3D) dynamics of the magnetic fields associated with the X9. 3 solar flare. We first performed an extrapolation of the 3D magnetic field based on the observed photospheric magnetic field prior to the flare and then used this as the initial condition for the MHD simulation, which revealed a dramatic eruption. In particular, we found that a large coherent flux rope composed of highly twisted magnetic field lines formed during the eruption. A series of small flux ropes were found to lie along a magnetic polarity inversion line prior to the flare. Reconnection occurring between each flux rope during the early stages of the eruption formed the large, highly twisted flux rope. Furthermore, we observed a writhing motion of the erupting flux rope. Understanding these dynamics is important in the drive to increase the accuracy of space weather forecasting. We report on the detailed dynamics of the 3D eruptive flux rope and discuss the possible mechanisms of the writhing motion.