Services, OAuth and a mobile app
First, this is a poorly worded question, so I'm answering the questions you asked, probably not what you really want though..

Both 3-legged and 2 legged work in Services with the OAuth module and the Services_oauth module enabled. I wrote a lengthy article on how to set this up here, and documentation here

But that doesn't really answer your question in terms of If I can help you get started with your flow. I suggest you ask a new question with the problems you're currently having rather than asking if we can help you get started on a solution we know nothing about.

This may help you understand why this is an extremely tough question to answer. Asking smart questions