why three RB’s i guess 2 would do if you need to have half the subscribers on one rb and the other half on the other. Just request your ISP for 2 public IP’s and have one IP on either
The IP assignment can be done dynamically by RADIUS while the load balancing will be done automatically between the two RB (the less heavy loaded will get the client requests)
I dunno all your specific requirements but maybe a consultant will give you the best ideas for your ISP