three.js is a webgl-based library that encapsulates some important tool methods and rendering loops in 3D rendering requirements. Run the 3D model in a browser and write code in js

unity3D is a game engine that can be written in js, C-sharp, and other languages, and then publish games to Windows, Mac, Wii, iPhone, WebGL (requires HTML5), Windows phone 8, and Android platforms. You can also use the Unity web player plug-in to publish web games that support web browsing for Mac and Windows.

three.js is a webgl-based library. My understanding is three +
js. Three means 3D and js means javascript. So taken together, threejs means to use javascript to write 3D programs.

Unity3D is a multi-platform integrated game development tool developed by Unity
Technologies that allows players to easily create interactive content such as 3D video games, architectural visualization, real-time 3D animation and so on. It is a fully integrated professional game engine. You can use programming languages such as javascripts and typescripts,C-sharp

unity was designed with cs architecture in mind, while browsers are bs architecture. If the application does not use many unity plug-ins, it can be used, because componentization is more convenient to use
complex scenarios, or use three.js. Although it is not componentized, at least the project is controllable.