As I see it, 3 rail modelers fall into three basic groups (no priority in the number sequence):
1. The toy train enthusiast. (Often will be subsets.)
2. The Hi Rail enthusiast (Can be subsets.)
3. The fastidious modeler that pursues a high degree of prototype fidelity. (Less subsets in this group.)
Group 1 will not be concerned with either prototype fidelity or other such concerns. Therefore, if they find a 36' car being offered is appealing to their criteria, they could be a small potential market for same. However, if they're closer to the subset spectrum that prefers colorful lithograph type cars (some even 4-wheel types), there will be little, if any, interest in a prototypical 36' car, which would be huge and loom over their tinplate or traditional sized 3 rail equipment.
2. These typically have scale equipment, but MAY mix-in traditional 3 rail models. This group may be a bit more particular as to era than group 1, but they could be more likely to be interested in a prototypical 36' boxcar.
3. This group will likely be concerned with prototypical accuracy in regards to era, perhaps even road name vs car type, etc. IF they are modeling the 1930s-1940s, then they would be inclined to be more interested in a 36' boxcar. IF they are modeling the far more popular transition era (roughly late 1940s to mid-late 1950s), then they would be more interested in 36' refrigerator cars, which lasted into the 50s in much greater numbers than 36' boxcars. IF they model one of the more the modern eras (1970 on), there will be zero interest in a 36' boxcar.
In view of the above, I suspect that the market for a prototypical based 36' boxcar would be very, VERY, slim in 3 rail.
All of the above is merely the way I see it and does not reflect any empirical data.
Andre