a baguette is a shape of stone, a type of cut. they're rectangular and small, generally... kinda like the bread, except for the size differential. you can have them as side stones, like in this setting (from niceice's site):
however, you can have all sorts of other cuts on your side stones, like round brilliant, pears, trapazoids, trilliants, emerald cuts, asscher cuts, bullets...
on the three-stone ring settings I've seen, I've seen three basic types. some rings have settings for each individual stone. in this case you end up with ~4 prongs per stone, or 12 prongs in all. some rings have shared prongs, so the prongs for the center stone are also prongs for the side stones (sorry, I don't know where to get a picture right now), so you have 8 total prongs. there are also three-stone rings I've seen where the side diamonds were set without any obvious prongs (perhaps channel or invisible set, I'm not an expert), so you just have the ~4 prongs for the center stone.
of course, instead of 4 prongs, you could have 6, like on aljdewey's three-stone ring with 6 heart-shaped prongs per stone, which gives you 18 total prongs.
why don't you look around at some rings first and see what grabs you, instead of worrying about the number of prongs? there is an "eye candy" thread in the Show Me the Ring folder which has quite a few rings, and a listing of places which have settings in the FAQ section.
hope I've cleared it up somewhat. I'm not an expert at all, myself!