Experimental evaluations that involve the educational system usually involve a hierarchical structure (students are nested within classrooms that are nested within schools, etc.). Concerns about contamination, where research subjects receive certain features of an intervention intended for subjects in a different experimental group, have often led researchers to randomize units at a higher level of the educational hierarchy. Existing work on two-level designs suggests that situations where contamination should lead to randomization at a higher level are likely to be rare. This article extends these results to the case of three-level designs. In order to understand the implications of mathematical results, existing information about the size of intracluster correlation coefficients (ICCs) in educational studies with three levels and about the extent of treatment effect heterogeneity across schools is discussed. Better empirical estimates of ICCs, treatment effect heterogeneity, and plausible contamination values are necessary to make full use of the results in this article. However, it seems likely that situations where contamination should lead to randomization at a higher level in three-level designs are rare.