So is it still going to be a tandem drive with a tag in front of the drives, or single drive with tag? If it is a third front axle, I would make a "U" axle. You can simply bend the axle tube or like this build, the center is box tubing and it is drilled for the axles.
If it is going to be a third axle you can rob the Peterbilt air ride from one of the snap kit for suspension. If there is lots of drive line clearance you can also simply use a straight axle. As far as steerable tags the ones that "I've seen" are lift axles not to say that they don't make non-lifting steers . Even doing a search of non-lifting axles, most of the results that come up are lifting axles. But steer axle is an option that you can simply use a front axle from most any kit. If it is going to be a third axle, I would definitely go air suspension, if not from a snap Peterbilt, then AMT, Revell, or Italeri will work too, or even the old Ertl Transtar air ride.
Another option, moluminum, and GW trucks offers a nice axles with suspension as well.