In these three sentences, they have the same meaning and all are correct. The only difference is that "I walked over the bridge" signifies that you were on foot and you could have "crossed" or "crossed over" in a car, on a bike, on foot, etc...

In English, you are more likely to say "I crossed the street" or "I crossed the road" when you walk from one side of the road to the other. People say "I walked over the bridge" more than they would say "I crossed the bridge" - both are correct, but culturally we are more likely to say it like I mentioned.