There may be a small percentage of people who do use 3D to design a MRR..But it's not necessary since the only dictating measurement needed is the radius of any 180 deg. return loop there might be in middle or ends of benchwork. IE. if, IF this curve is 22"r, diameter is thus 44". Bench then needs to be no less than 48" wide insuring track to be a safe 2" in from edge on either side..The length of bench is dictated by allotted room size..And keeping grades below 3% (and closer to 2%) will dictate how long each grade's 'ramp' is going to need to be to accomplish such..
What 3D does (virtual, or miniature model of model) is show certain 'esthetics' of the design, but is not necessarily, if any more helpful than a pencil and paper in designing a train layout; its confines, measurements, tolerances, and total materials needed, for the project to materialize as planned..