You may find that the app will need to be written more than once, for it to work properly across all platforms. Depending on what it is you're trying to achieve, I know of some tools where you can make an app for iphone and have it automatically portable to android, but am not sure these would be particularly useful for all types of app, or for crossing over to blackberry. Typically as-well there will need to be some tweaking for each platform to make it run well, but it is of course very doable.