[dwb]

When I was looking for some girder bridges, I trawled the major plastic model kit manufacturers - Faller, Vollmer, Kibri. They all do some form of curved bridge sections though whether they would go out as far as 3 feet, I doubt very much. It's possible Roco might do some high level support kit that goes to 3 feet as their set track range has large radius curves in it.

I would be tempted to build the track bed as if it were a 3" (or whatever) wide fixed baseboard and then build the bridge around it.

I didn't find a suitable girder bridge..

David