The purpose of "The State of Computer Science in Illinois High Schools Series" is to analyze the landscape, structures, and pathways of computer science (CS) education in Illinois and to create a baseline by which to measure the expansion of CS education in the coming years. Beginning in the 2023-2024 school year, all districts in the state that serve grades 9-12 must offer every student the opportunity to enroll in a CS course. Because not all districts in the state had CS offerings before this school year, it is imperative to measure capacity for, access to, participation in, and experiences in CS education (i.e., CAPE framework) before and after the mandate went into effect. Analyzing trends through the lens of the CAPE framework will highlight progress while identifying existing gaps in providing equitable access and outcomes for all students. The first report of this Series provided an overview of the CS education landscape in the state by analyzing overall participation trends and details about the most enrolled CS courses. The second report analyzed the CS student body, focusing on students from historically marginalized backgrounds, including trends of their participation in general and rigorous coursework and course outcomes. This third installment of the Series shifts focus to the CS teacher workforce. Because Parts 1 and 2 focused on overall trends in the CS education landscape and who enrolls in CS coursework, a report that focuses on teachers is essential to understanding if Illinois has the capacity for CS education in its high schools and if students are receiving a high-quality, equitable CS education. This report analyzes the characteristics of CS teachers in Illinois high schools, including describing the CS teacher workforce by their gender and racial make-up, what licensure and endorsements they hold, if they are appropriately assigned to CS courses based on their endorsements, if their characteristics change when disaggregated by different CS courses, and their CS workload. Similar to previous reports in this Series, this report ends on a Spotlight Analysis to explore the untapped teacher workforce in the state.