Hi there, right...
1) Installing any expansions will not impact any of your saves as these are kept in the My Documents folder under EA>The Sims3. However, as a precaution, we always advise to backup your saves elsewhere just to be on the safe side.
2) All 3!
Seriously though quick pros/cons of each expansion. So, my personal favourite is World Adventures. I love the tomb raiding, the 3 new world (stress here that they are vacation world, so you can only stay for a set amount of time) are stunning, jaw dropping stunning. And the adventures are just plain fun. However, it can be a one or two time only thing as I can see it can get boring playing the same adventures over and over with different sims. However, for the worlds, definitely get it.
Ambitions adds that whole extra dimension to jobs, which for me slotted right into my gameplay. Lots of subtle differences, new town (Twinbrook), plus the big obvious changes of having new jobs. Late Night, I've just got it so i'm still learning all there is to it, but all I've got to say is vampires (big fan), and I do love all the extra stuff you get, the different feel to the game, etc.
3) Your default town can be any town you want, cause there is no default town as such. With your save games you will remain in that town. Starting a new game you get the option to choose. Also there is the option to place the new lots that come wtih Ambitions and Late Night into your town. So you will get the option to place the consignment store, laundromat, junkyard, etc from Ambitions into Sunset Valley, same with the clubs. Or you could ignore that bit, all up to you.