There are some apps in android that you may use like "Terminal Emulator" , "Terminal IDE", when connected to Raspberry pi in same network (for this you can use USB tethering in android). Most linux command will work because raspbian OS itself is kind of linux. 

Hope this will help.

In this case Nodered would be an intuitive option for you. There are apps available on android platform to perform MQTT communication and same can be done on your RPi3 as well. One of the examples is Paho developed at CDAC labs (although I am not sure if it is on playstore yet).

Nodered works on both windows and linux platforms so even if you want to add aother node it would not be an issue with RPi.

This might be helpful

http://noderedguide.com/

Please go through these links

www.instructables.com/id/Raspberry-Pi-Android-App-communication/

https://www.stavros.io/posts/writing-my-first-android-app-control-your-raspberr/

i doing my project in web usage mining using web log files,

i did not know how to preprocess log files in text formate and how to convert text log into json formate?

follow this link

stackoverflow.com/questions/244777/can-comments-be-used-in-json