In Australia, once you get more than two bikes on a horizontal rack you will be going over the 60% wheelbase rule too. I think if you need more than two bikes a vertical rack is the way to go. It does depend on the vehicle but sometimes any rack will exceed 60% of the wheelbase and technically not be legal.Three bikes yes, but not more on a tow ball because of the weight and leverage on the bar with a heavy bike three feet behind the ball.
I always put the heaviest bike on first, lightest one last, and if possible take anything easily removed off, battery (if possible), drinks bottles, lights, stuff like that.