The first years of teaching are crucial for novice teachers' integration into and retainment in the education system. The support they receive from experienced teachers impacts their professional development. Science, technology, engineering, and mathematics (STEM) teachers require specialized support from domain-specific mentors. In this study, we examined how a three-level mentoring support system contributes to STEM novice teachers' professional growth (PG) and to their mentors. The support system levels are individual mentoring, group mentoring, and mentoring networks. Based on the framework of teachers' professional development, there are three PG dimensions: personal, professional, and social. Our research goal was to analyze teachers' professional growth by the various mentoring level and dimension combinations. The study, conducted using a mixed methods approach, included 123 novice and 78 experienced STEM teachers. We examined the novice teachers' perceptions of their teaching efficacy, the mentoring factors, correlations between the professional growth dimensions, and the contribution of each support level to the growth dimensions. We found that experienced teachers perceive novice teachers' efficacy as lower than that perceived by the novice teachers. We identified gaps between the mentoring factors described by novice and experienced teachers and a strong correlation between the growth dimensions. All three mentoring support levels facilitate substantive personal, professional, and social growth. Individual mentoring contributes the most to all three growth dimensions, followed by mentoring networks. The contribution of this research is its elucidation of the intertwined support levels, which provide scaffolds for the novice teachers and facilitate the growth of the experienced teachers.