360° videos provide an immersive experience, especially when watched in virtual reality (VR). Yet, even though the video data is inherently three-dimensional, interfaces to access datasets of such videos in VR almost always use two-dimensional thumbnails shown in a grid on a flat or curved plane. We claim that using spherical and cube-shaped 3D thumbnails may provide a better user experience and be more effective at conveying the high-level subject matter of a video or when searching for a specific item in it. A comparative study against the most used existing representation, that is, 2D equirectangular projections, showed that the spherical 3D thumbnails did indeed provide the best user experience, whereas traditional 2D equirectangular projections still performed better for high-level classification tasks. Yet, they were outperformed by spherical thumbnails when participants had to search for details within the videos. Our results thus confirm a potential benefit of 3D thumbnail representations for 360-degree videos in VR, especially with respect to user experience and detailed content search and suggest a mixed interface design providing both options to the users. Supplemental materials about the user study and used data are available at https://osf.io/5vk49/.