Although i realise that the three services have significantly differing characters i would advise any person seriously considering a career in military aviation to apply to all three. With the slots getting more scarce i think you need to load the bases in your favour.
It won't blot your copy book if you tell one service that you've applied for another, actually it will probably be taken as a sign that you're committed. (Obviously tell them that they're the favourite and the others are just an insurance policy.) If you're a university leaver time is not on your side and concurrent applications will save the time you don't have. Three consecutive applications could take years!
At the end of the day whichever machine you end up driving around in, and whichever unifrom you end up wearing, you'll have a great time.
One word of warning though. The chap on the other side of the desk may well have some quotas to fill. If you want to be a driver then stick to your guns. Never accept any other role, no matter how appealing they make it sound, until all three services have said no to you having a stick!