That really depends how much your designs are in common for the different platforms. You probably should define which is your "lead" platform one that you finish the design on and then adjust for the other ones. Which is probably easier to do if you have different files for each design. You should also consider that you will need multiple views for each platform, so doing it all in one file will become chaotic fast.
You might be able to re-use some assets for iOS in Android or the other way around. But you will only be able to tell after you are done designing them.
There is no perfect answer to this question as apps and workflows are so different. You should look at this from a planning point of view:
- Do you have a screenflow?
- How many screens will you have to design?
- How similar will the designs be?
- Can you make a styleguide or will you design each screen for every
- What will you hand over to development? colours and measurements or
9-patches and slices?
- Can you separate the design form the assets completely?
Cross-platform design is a lot more about planning than moving pixels around.