RuneScape 3 Mobile app was releatsed on Google play store as limited beta. Only first 25,000 players are allowed to install.
If you are not one of the luckly players to get in during inital beta offering, there is an unoffical way to get RuneScape 3 Mobile Application by downloading APK file from Google Play store and installing.
To do this, you need a software called YalpStore. This is Open Source software available from GitHub
Download the APK file to your mobile phone. Next you need to install the YalpStore APK file.
To install APK file, you need to enable installation of application from “unknown sources”. This can be done under Settings > Privacy > Unknown sources = Enabled.
Once APK installed, start the application. Search for “Runescape 3”. You will see the RuneScape 3 App, you can download and install.
Here is some images of RuneScape 3 Mobile App
Player Owned Farm