When I flew for Express Airways (1972-1993),the company had several Tri-Gear conversions of mostly C-45's with R-985's that they picked up cheap in the late 70's.I think that they came from a small airline that did tours of the Grand Canyon.I never flew any of them,but did fly about 300 hours in a Volpar VTB-18 (Volpar Turbine Beech).This was a kit conversion with Tri-Gear and TPE-331 turbprops that was originally used by Air America.Express Airways bought it from Trans Mountain Airways in Denver.
The point is that Volpar was converting conventional gear Beech 18's long before Beech produced them as an option for new manufacture.According to the following link,Volpar stared their operation in 1960.
http://www.ascendant-online.net/main/in ... icle&sid=3