Early childhood mathematics education takes attention for decades. The effect of qualified mathematics education during early years has an impact on children's future academic success. Main purposes of the study were twofold; to examine children's current 3D geometry thinking skills and to investigate the development of children's 3D geometry thinking skills after a 3D training program developed by researchers, and named as 3D in Early Childhood (3DinEC). The qualitative research design with convenience sampling method were utilized. The participant children were three girls and four boys with the age range of 60 months to 72 months. The data were collected using a semi-structured interview form including six abilities (Pittalis and Christou, 2010), and 15 items. In the analysis of the data, descriptive and content analysis methods were used. Findings indicated that participant children had a limited understanding regarding 3D geometric thinking. It was also found out that some of these thinking skills like identification of 3D geometric shapes and recognition of these shapes' properties, etc. could be enhanced through various activities.