Studies of mathematics teacher preparation frequently lament the divide between the more theoretically based university methods course and the practically grounded classroom field experience. In many instances, attempts to mediate this gap involve creating hybrid or third spaces, which seek to dissipate the differences in knowledge status as individuals from the university and from K-12 classrooms work together in support of prospective teacher (PST) learning. However, what is missing in the literature on these third-space enactments is an exploration of the contributions of different contexts (i.e., methods and the field) to PST learning and an articulation of the synergistic knowledge arising in the third space. This exploratory study draws on Lampert's three-pronged teacher-child-content model to examine the possible contributions of elementary mentor teachers (MTs) to the learning-to-teach-mathematics experiences of PSTs. More specifically, we focus on a third-space learning context in which university-based teacher educators, MTs, and PSTs collaborated to conduct and analyze task-based problem-solving interviews of elementary children. Our analysis identified ways that MTs could potentially enhanced the learning-to-teach context as well as moments when MTs' contributions introduced problematic ideas about children and teaching. Finally, we explore the benefits and complexities of leveraging these MT contributions to create a third-space learning opportunity.