We are currently trying to start a project with three different OS's, iOS, Android, and Server components. The argument has been on how should we create the stories, should there be a story for each OS and point each story, or should we create one story for all three OS's and point that one story. Note: It's the same feature across the three different OS's, and there are different devs working on each OS.
You should write the same user stories and acceptance tests for all the three Operating Systems. By the way, since you have three platforms and three different groups of developers working on each of them, you will have different effort estimations (story points) for the same story for each OS. For this reason, I suggest you to keep track of the progress using three different task boards.