KEY CLINICAL MESSAGE: Immune checkpoint inhibitors can rarely lead to occurrence of myositis, myocarditis, and myasthenia gravis (MG). Early recognition and multidisciplinary management are crucial for optimal outcomes. Vigilance for overlapping toxicities is essential in patients receiving combination immunotherapy. ABSTRACT: The use of immune checkpoint inhibitors (ICIs) has revolutionized cancer treatment, but it is associated with immune-related adverse events (IRAEs) affecting various organ systems. The simultaneous occurrence of MG, myocarditis, and myositis highlights the complex nature of IRAEs. Early recognition and comprehensive multidisciplinary management are crucial for optimal patient outcomes. We present a unique case report of a 76-year-old male patient with advanced melanoma who developed concurrent myositis, myocarditis, and MG while receiving combination immunotherapy with Nivolumab and Ipilimumab. This case underscores the significance of recognizing and addressing the "Terrible Triad" of IRAEs in patients receiving ICIs. Healthcare providers should maintain a high index of suspicion for overlapping toxicities and promptly initiate appropriate interventions.