James,
I found that staggering the applications helped when I approached this. I was also a UAS chap so I naturally went to OASC first, then the AIB, then RCB.... then OASC again...... and got it! This was within a year. I found that I learned something on each board visit that I could then apply to subsequent board! I think applying to all three at once would create masses of work for you as you have to gen up on three services at once.
At the moment I'm prepping to start IOT in May, so the "pro's" of the RAF over the RN or Army as far as I'm concerned are:
1) the RAF take many more Pilots (approx 140-160 this year I'm told
)
2) IOT is considerably shorter (7 months vs. 12 months) and you're not wasting time learning a skill that you won't use i.e. boat driving or soldiering.
3) Greater chance of something other than helos once you start flying.
4) Being ex-UAS I knew that I liked the RAF... mostly.