There is a considerable body of research on children's number knowledge development, and in particular on children ages 4 years and older and who have some counting experience. Children in early care preschool settings (ages 2 and 3) and who are not yet experienced counters are an understudied population. We conducted a multiple-case study on five 3-year-olds over the course of a year, investigated their challenges and successes as they moved through the developmental progressions for specific number knowledge domains, and determined their key shifts in learning as they related to early predictors of later mathematics achievement. We found interesting variations in their test scores and, with multiple data sources, determined key shifts in learning observed across cases including acquisition of verbal counting skill, one-to-one correspondence, cardinality, and connecting and coordinating their number skills. While the developmental progressions provide teachers with a framework for understanding typical mathematical development, every child's learning path is different. This research provides an in-depth look at 3-year-old preschool children's variability in number knowledge and their key shifts in learning as they relate to early predictors of later mathematics achievement. We share implications for how and when early preschool teachers can support learning.