When I went to my first interview, the Captain who was giving it went over my 3 choices that I had on my application and discussed them with me in detail.
He told me which ones I would have to wait a very long time to get into, because apparently some trades have a very low turn over rate, and are competitive to boot, and that those who already have 3-5 years on the list have priority over someone just coming in.
He told me that the interview was the last stop for choice of trades, if I had 3 on my list, they would offer me whatever came up first.
If the 3rd choice comes up and you get an offer, but you decline because you would prefer to wait for an offer from your first choice, then you will likely never get in, because they will be pissed off at you. So pick trades only that you are seriously interested in, even if its only 1. You still have a chance to tell the interviewer that you have changed your mind about your trades.
Plus I asked him if I went infantry, but wanted to switch over to another trade later on, how difficult would it be? He ended up getting a little angry at me, saying that from his "Human Resources" point of view, why would he want to waste funds training me from scratch twice? Which obviously makes sense, so don‘t ask about that.
*The recruiter told me that people do it, and that infantry personel are desireable to the other trades based on their military experience, and work ethic. so thats why I asked.
Basically the interviewer is the best person to ask about what kind of chances you have for getting into a particular trade, because there is a lot of variables. Some depend on you, and some depend on the current state of whatever trade.