WHOA! u wanna know the difference?
about 1-2k. thats the difference. hahah
THe 17-40 is an L lens. meaning its of higher quality in terms of built and IQ.
The 17-55 IS is an overpriced lens which i dont recommend u getting unless ur some rich-ass kid with nuthin better to blow ur cash on, and dont want to donate to our LYN grass eating club.
The 17-50mm is slightly noisy and has a slightly less focal range than the 17-55 is. but the performance is there. stunning IQ. plus. its way cheaper than either of the other 2 lenses. 

issit that bad? actually im considering to get it...
never try myself, but read review said it have L lens IQ with IS, just the built quality is poor.
pls enlighten me....

if 3.5k is cheap then go ahead. its not even an L lens man...
PRICE
Maybe ur confused. the question was the Canon 17-55mm f2.8 IS lens, not the 17-55mm kitty lens la...