I think stage 1 is fine, but to make it slightly easier for the attackers (so they win 75% of the time) I'd suggest changing the entrance to the upper platform. Right now defenders can access it as easily as defenders until the cart is by the bridge. It should be far easier for the attackers to control it than the defenders, similar to Dustbowl stage 1. To do this you need the entrance to it moved further away from the red spawn. I'd just block off the entry from that side tunnel and instead have the entrance come in only from the first main area. Since I don't have an overhead image of the map, I'll describe it better using this:
http://image.hazardstrip.com/ss/maps/141583.jpg
I'm thinking the entrance would be on the right side of this area behind the Brown Nugget (heh) building. Another option would be to just put a 1-way gate on the part of that hall that defenders use to go up. That would allow attackers to still use the hall to go directly to the bridge, but defenders would lose their easy access to the upper platform.
On stage 2 FLOOR_MASTER's comments are pretty much what I would say as well (although I already chipped in additional info).
For stage 3, the shortcut hall doesn't help much. How about having it exit into the building on the left and having the building positioned closer to the shortcut wall to accommodate (or just make the building longer)? Right now the stage works pretty well until you get to the choke point, which is between the shortcut's exit, the upper exit, and the end of the shallow trench. I think having the shortcut a little more protected would help the attackers just a bit while still making it hard for them to take the area.
Another minor note... that small trench on stage 3 always bothered me. It doesn't really look right and there doesn't seem to be any purpose for it. To incorporate it more into the gameplay, turn it into a tunnel entrance that just leads into the trench right below the final cap. I'd suggest instead of making it look like a cave, make it look like a basement entrance made of concrete and put some open doors on it. This would also help attackers just slightly since they'd have a 3rd way into the final room, and it'd be a bit protected from sentries up above.
Final note: how about an overhead image of the map? I'll do some drawings on it to make my comments more clear.
