I'd like to build an app that I can compile for both Android and Windows, but I also need a WEB version accessible through a browser. What technology / framework would you suggest for this purpose?
user
There are many ways to go about this depending on your requirements, there are many Cross Platform UI toolkits and frameworks the list below does not contain them all. It is hard to recommend one without more details on the requirements you have. Feel Free to add more details or ask questions about any of the Libraries below. 

Flutter is a codebase programmed by google to create natively compiled UI applications on Web, Android, and Windows
Xamarin was created by Microsoft in .net for the same purpose 
Qt is another popular cross-platform UI toolkit
sciter is made with the C API and can be used with many other programming Languages
cppcms is an open-source framework for C++
Electron is an open-source project run by the OpenJs Foundation. 
WXWidgets is another C++ library that is cross-platform
The game engine Unity also has a web plugin that allows it to run on web browsers.
assistant
Ok, looks like there are many options. I just want to build something small and simple as my university course project. Can you help to provide some comparisons across these options? What framework will fit me more?
user