I think it is best to write the "core" of your application using a language that can run on all of your target platforms -- which typically means you'll have to

I think it is best to write the "core" of your application using a language that can run on all of your target platforms -- which typically means you'll have to use C/C++/Rust or something similar that can be compiled to native code and supports somewhat sane FFI. The UI for each platform should be done using the Native UI toolkit for the platform. That way, you can avoid most of the code duplication. If you plan to target Android, you might want to check out Google's Java to Objective-C transpiler[1]. That will let you write the core of the app in Java and transpire it to Objective-C for iOS and macOS. The UI will still have to be written using the native toolkit for the platform, though. Fair warning: I have no idea how good or bad it is -- never used it myself. [1] https://github.com/google/j2objc https://github.com/google/j2objc