It’s already possible to write for those 3 with Kotlin although for desktop you are stuck with Java UI components (e.g. Java Fx) instead of native UI components.
Intel created the Multi-OS engine (https://multi-os-engine.org) that basically lets you write for iOS with Java and thus also with Kotlin. Here are some sample apps using Kotlin with MOE: GitHub - multi-os-engine/moe-samples-kotlin: Multi-OS Engine: Kotlin Samples.
It is of course also possible to write for those and more with C# (or any CLR language) using Xamarin, but Kotlin is a better language, which is why I advocated in another thread for Kotlin on the CLR instead of native.