I have not used a 24-70, but I have used a 28-70 (and own a Tamron 28-75). I have used both the 17-35 and 17-55.
My opinions, FWIW:
- All 3 are good lenses
- I don't think 24mm is quite wide enough for event photography
- I don't think 35mm is quite long enough for event photography
- 17-55 is the "least good" of the 3, however it is still very good
- I recently borrowed a 17-55 for my aunty's wedding, and believe it is the perfect zoom range on a crop body for event/wedding photography
- I'm going to get one

I reckon if you're going to do more weddings than portraits, get the 17-55.
If you are going to do more portraits than weddings, get the 24-70.
YMMV.