Will an app made for windows store support WindowsRT, windows8 and windows 8 mobile? [closed]

Question

I am very much confused about these.

I would like to develop app for windows 8 , Windows RT , and windows mobile. I have windows 8 OS installed in my laptop.

As far as I know, Windows RT is for ARM based devices.

HERE ARE MY QUESTIONS:

1. I develop an app, and put in windows store. Will that can be downloaded from a ARM based device ,which runs on Windows RT, AND from a windows 8 laptop , and same from a Windows 8 mobile?

OR should it be different apps ?