While computer science is widely recognized as crucial for all U.S. high school students, challenges persist for teachers' pedagogical, content, and culturally responsive-sustaining education. In this paper, we explore how a co-teaching model with an experienced CS instructor shaped three novice CS teachers' knowledge while implementing Advanced Placement Computer Science Principles courses. Using a multi-case study methodology, we analyzed semi-structured interviews, content knowledge test, and teacher created artifacts from the three teachers to compare their knowledge and experiences. We found that the teachers perceived the co-teaching model as valuable professional development, used pedagogical strategies to fill in content knowledge gaps, and struggled with culturally responsive-sustaining education. We discuss the implications of these findings for supporting novice CS teachers.