Where to start to develop a cross platform App that can be used in Windows, Android, and Apple?

First off; I am completely new to this aspect of programming and only have limited experience in Javascript (primarily for PDF scripting). I originally started out with the hopes of developing a full fledged program in Unity, but I am thinking smaller now. Neither the full fledged app or the cross platform app are actual games but more game aids for table top games (non video games).

Now for the app. For starters I would like to create a simple HP (hit points) and PP (power points) tracker. I want to add a few more functions as time goes on with a drop down list for actions and an initiative tracker. Basically the end user will enter in their basic HP and PP numbers. Then they will have an option to either add or subtract from those numbers (max being the initial number they put in, but can increase later).

If there is a better way than using Unity for this then please point me in the right direction otherwise I will need help with this. Tutorials will help as long as they are slightly focused on what I am trying to do. I have the android developers kit but I need these programs to be cross platform capable so that only works a little.

I hope I made the right choice by choosing Unity,

Thanks everyone in advance~