Bart van Ingen Schenau's answer was excellent.
I will only add to his response by stating that essentially the original story was too big to be a story and was, in fact, an Epic or Feature. So what you're doing is more in line with what we call "vertical slicing", to create a "right-sized" story that could be completed within a sprint.
You mention that "we use to split by platform and create a different card for each one, but when the sprint finish, and only two of the three cards are done, I can't mark the main card as "Done" (at least I think so) and feels like we are delayed."
You seem to be under the impression that, unless all three platforms are ready simultaneously, you could not release. But why not? As Bart says, if 80% of your customer base uses Android, and you can finish that in a sprint, you could release that new functionality to 80% of your customers after one sprint. Therefore, you are behaving in a way in keeping with the agile principle of releasing frequently.
Conversely, if you try to keep large stories and won't release until the whole thing is done, you delay release from 2 weeks to 6 weeks. That runs contrary to the agile principle.
I get the impression that your discussion of "splitting stories" may be driven by the functionality of the application you are using (such as Rally) to track your work. If that is the case, and you are using a splitting functionality native to the tool, be careful to not allow the tool to determine the process. Remember: Individuals and Interactions over Process and Tools. I have seen many times that scrum masters begin to make decisions on how to handle topics like these based upon what their tool does, rather than what makes sense from an agile perspective.
we typically would have created an Epic or Feature "News Feed" that encompassed the three platforms, and individual stories for each platform. If, however, you need to get all the Andriod work out first, and this would leave many "partially complete" Features in the backlog, and for some reason this causes heartache in management, then we would create separate Epics by platform so we could demonstrate progress and completion of those features by platform. That way, we are working in a way consistent with agile principles and values while using the tool's functionality to our advantage.