Looking for tools to build a cross-platform and cross-OS application

I have a fairly simple project I want to make that I'd like to build for Windows, Android and Linux. I recognize that I'll need to make different UI for desktop and mobile, but aside from that I'd prefer to not have to re-create functionality for different systems. I have a few years of experience coding but only for game dev, and I'd like to make a normal desktop/mobile app. What are the best tools available for doing this?