No, it'll learn all three, but the one you use the most will obviously become more natural to the players sooner.
I tend to start the season with what I consider my two main tactics, then when they're familiar enough, I add a third if I feel it's necessary. Keep in mind though that 1) there's no point in aiming to get 'full' tactical awareness too soon (not to the extent where your players suffer attribute loss as a result of a major lack of their normal training) and 2) tactics matter less in FM2013 - more important is the role of players suiting their attributes, the balance of the team and countering the opposition's tactics. "Magic tactics" don't give the rewards any more.
My normal pre-season is to stick the players on a very high fitness programme the moment the previous season finishes (I know they're usually on holiday, but it's set ready in case I forget). Then after 2-3 weeks of pre-season I'll move it onto a team cohesion for a week or 2. After that, it's back to the usual - primarily attaching with a smattering of other training every now and then. Match preparation will be quite high until the pre-season is over, at which point I'll move the slider down gradually over the first 2 or 3 months of the season. Match prep is generally set to attacking movement, though that changes depending on how my team is defending and what games are coming up.
It really shouldn't start from scratch each season though. Players are dumb, but not dumb enough to completely forget the game over a couple of months. Obviously if you overhaul your squad over the summer to any great extent then it should require more work, but it shouldn't need starting all over again if your squad stays consistent.