Hold the phone on those bridges, because they may be making some changes to the run course. Best case scenario, however, is 2 bridges out and 2 bridges back.
I can't really find good info to give you about the grade on these bridges, but what gets you isn't really the grade. Two of the three bridges are drawbridges with steel grating, and you'll be crossing a drawbridge somewhere in the final miles of your marathon. Good place for a walk break if you're tired... a good running buddy of mine was on a relay team, and she was splitting for a 3:15 run when she fell on the bridge at mile 25. She didn't finish... instead she got an ambulance ride to the ER with some significant facial trauma.
The first bridge, within the first mile of each loop, is the Thomas Rhodes bridge. It's the steepest, but not a drawbridge... maybe 100 feet high? This doesn't do it justice because it's from so far away, but here's a pic...
http://www.flickr.com/photos/thundersnow/9604981/
The second bridge is the Isabel Holmes bridge, which is the less-feared of the drawbridges because it's a more gradual slope both up and down, and a shorter span. The metal grating is the worst part of it.
http://www.flickr.com/photos/thundersnow/9604819/ (also a long shot)
The third, and perhaps most-feared bridge, is the Cape Fear Memorial Bridge. It's got a long climb up an on-ramp to get on it, and then you have the steel grating at the main span. Here's a pic from the top of the ramp:
http://www.flickr.com/photos/thundersnow/9605162/
And here's someone else's during-the-race view from up on the bridge, so you can see the grating that you'll be dealing with:
http://www.flickr.com/photos/goldeneyep ... 994373422/