This package provides a fast, interactive cross-platform, and easy to share WebGL'-based 3D brain viewer that visualizes FreeSurfer and/or AFNI/SUMA surfaces. The viewer widget can be either standalone or embedded into R-shiny applications. The standalone version only require a web browser with WebGL2 support (for example, Chrome', Firefox', Safari'), and can be inserted into any websites. The R-shiny support allows the 3D viewer to be dynamically generated from reactive user inputs. Please check the publication by Wang, Magnotti, Zhang, and Beauchamp (2023, <doi:10.1523/ENEURO.0328-23.2023>) for electrode localization. This viewer has been fully adopted by RAVE <https://openwetware.org/wiki/RAVE>, an interactive toolbox to analyze iEEG data by Magnotti, Wang, and Beauchamp (2020, <doi:10.1016/j.neuroimage.2020.117341>). Please check citation("threeBrain") for details.