Double tearing modes in ultra-low-q equilibria have been simulated by three-dimensional MHD calculations. Spatially localized MHD relaxation is shown to be driven by double tearing modes, which bring the field configuration in the unstable region to a Taylor-like relaxed state. When the pitch minimum of the initial state is located near the centre of the plasma, the central value of q decreases rapidly and the outer magnetic island replaces the original magnetic axis. When the pitch minimum is located near the wall, the central core region remains stable and reconnection occurs in a limited intermediate region.