Vulnerability of power grid is a critical issue in power industry. In order to understand and reduce power grid vulnerability under threats, existing research often employs defender-attacker-defender (DAD) models to derive effective protection plans and evaluate grid performances under various contingencies. Transmission line switching (also known as topology control) is an effective operation to mitigate outages or attacks. In this paper, we include post-contingency transmission line switching operations into a defender-attacker-defender model. To solve this complicated tri-level formulation, we customize and implement an exact algorithm, i.e., nested column-and-constraint generation (NCCG) algorithm, to compute global optimal solutions. We perform a set of numerical experiments on IEEE RTS one-area system (1996), demonstrate the significant improvements from transmission line switching, and highlight that hardening plans derived from this new DAD model are very cost-effective compared to those obtained from traditional DAD model.