I am making a three tiered cake and icing with buttercream and covering with sprinkles (jimmies/nonpareils).  I want to stick fondant letters and decorations to the sprinkles.  Will they stick or will they fall off?

With a little piping gel on the back of the fondant, I don't see why they wouldn't stick.  I would just make super sure that the sprinkles you adhere to the buttercream are really pressed on/into the icing and that the fondant pieces you attach aren't too heavy.  But if it is just things like thin letters then you should be fine.

I agree    Roll that fondant as thin as you can & still hold it’s shape then let them dry well before applying them   

if the fondant pieces are on the sides, hanging vertically, I would remove some of the sprinkles from that area so you are sticking fondant to buttercream in some spots on the back anyhow -- you could also make paper patterns of the letters and stick them on the buttercream with pins or toothpicks so the sprinkles won't adhere there too -- maybe make the pattern a little smaller so enough jimmies get onto the buttercream so it looks seamless once you apply the fondant pieces -- that will allow for a nice region to be able to adhere there -- 

just depending on how big/heavy the fondant pieces are -- speaking of toothpicks, you can put a couple toothpicks under the letters to hold them up -- you can push them in flush -- or you can leave them sticking out so they can be removed later -- but that's only for items placed on the sides of the cakes 

K8 why do you always have to come up with the right thoughts that the rest of us miss?? LOL!  Of course that makes scense.  Just remove or cover over the area(s) where the fondant pieces will go -- especially the bigger the fondant piece. 

hahahaha -- you're so funny, lynnie 

to answer your question -- because I've screwed up everything at least once if not multiple times and learned the hard way

...eventually...