Plans changed along the way, likely when Microsoft started to notice that most users were not interested in the Paint 3D app or 3D content creation. Microsoft's initial plan was to replace the popular Paint application with Paint 3D, but keep Paint alive as a Store application for those who wanted to continue using it. My verdict back in 2017 was that it was a niche feature that Microsoft blew out of proportions. 3D Viewer could be used to view 3D models in the application.
It looked like a crippled version of Paint, but with 3D as its main focus. Microsoft introduced the two applications in the Creators Update, which it released in 2017, two years after the initial release of Windows 10.
