partial class Examples
{
  public static Result PlanarSurface(RhinoDoc doc)
  {
    Point3d[] corners;
    var rc = Rhino.Input.RhinoGet.GetRectangle(out corners);
    if (rc != Result.Success)
      return rc;

    var plane = new Plane(corners[0], corners[1], corners[2]);

    var plane_surface = new PlaneSurface(plane,
      new Interval(0, corners[0].DistanceTo(corners[1])),
      new Interval(0, corners[1].DistanceTo(corners[2])));

    doc.Objects.Add(plane_surface);
    doc.Views.Redraw();
    return Result.Success;
  }
}

import Rhino;
import rhinoscriptsyntax as rs

def RunCommand():
    rc, corners = Rhino.Input.RhinoGet.GetRectangle()
    if rc != Rhino.Commands.Result.Success:
        return rc

    plane = Rhino.Geometry.Plane(corners[0], corners[1], corners[2])
    u_dir = rs.Distance(corners[0], corners[1])
    v_dir = rs.Distance(corners[1], corners[2])
    rs.AddPlaneSurface(plane, u_dir, v_dir)

if __name__ == "__main__":
    RunCommand()