Does doing a transformby() in the instance give the desired results?
Arc3p arcp3p1 = new Arc3p(new Point3d(0, 0, 0), new Point3d(50, 25, 0), new Point3d(100, 0, 0),true);
arcp3p1.TransformBy(ed.CurrentUserCoordinateSystem);
btr.AppendEntity(arcp3p1);
tr.AddNewlyCreatedDBObject(arcp3p1,true);