Despite distinctions in molecular architecture, various block copolymers self-assemble into line-and-space structures with similar topological defects. To reduce the number of defects in the course of directed self-assembly (DSA), it is essential to understand the impact of molecular architecture on the stability and annihilation mechanisms of topological defects. In this work, we study a prototypical dislocation defect in lamella-forming ABC triblock copolymers. By varying the molecular architecture (e.g., the composition and the incompatibility), we examine the thermodynamic properties of defects by self-consistent field theory (SCFT) and we investigate defect-annihilation mechanisms by the string method. Our numerical results reveal that there exist multiple annihilation paths. By altering the molecular architecture, we modify the free-energy barrier of defect annihilation and thereby select the most likely annihilation path on the free-energy landscape. Therefore, defects with a similar topological structure in systems with different molecular architectures may annihilate differently. Our investigation indicates strategies to minimize the defect density by choosing an appropriate molecular architecture.