Reduction engineering in Beulah Montanna has the 3d scale castings. It is going to be tuff finding anything that will have exactly what you want. You might consider a piece of thick wall tubing and try to find a 6 spoke wheel and cut the rim off. That would get you closer to what you want. You can get the tube most any size you want and 3" don't seem like it would be that much. Finding a wheel might be a little harder.
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Ebay has some heavy cast iron gears that might be perfect. If you could find the right one just turn the teeth off and shrink a piece of tubing on. It would be real close to what you want.