Timelines are common visualizations to represent and manipulate temporal data. However, timeline visualizations rarely consider spatio-temporal 3D data (e.g. mesh or volumetric models) directly. In this paper, leveraging the increased workspace and 3D interaction capabilities of virtual reality (VR), we first propose a timeline design space for 3D temporal data extending the timeline design space proposed by Brehmer et al. [7]. The proposed design space adapts the scale, layout and representation dimensions to account for the depth dimension and how the 3D temporal data can be partitioned and structured. Moreover, an additional dimension is introduced, the support, which further characterizes the 3D dimension of the visualization. The design space is complemented by discussing the interaction methods required for the efficient visualization of 3D timelines in VR. Secondly, we evaluate the benefits of 3D timelines through a formal evaluation (n=21). Taken together, our results showed that time-related tasks can be achieved more comfortably using timelines, and more efficiently for specific tasks requiring the analysis of the surrounding temporal context. Finally, we illustrate the use of 3D timelines with a use-case on morphogenetic analysis in which domain experts in cell imaging were involved in the design and evaluation process.