the best way to connect your ROSbots together (even if they are in different networks), is to use our software product- Husarnet (https://husarion.com/software/husarnet/) . It’s basically a P2P, VPN network for robots.
All you have to do is create a Virtual Robot containing your 3 ROSbots and (optionally) your laptop. All devices connected by Husarnet into a single Virtual Robot see each other, and you can access them by using their hostname or IPv6 address. Here’s a tutorial that will show you how to use Husarnet with a simple robot based on CORE2-ROS (the same controller is in ROSbot) - https://husarion.com/tutorials/husarnet/following-object-using-your-smartphone/ .
Please let us know your impression on how it performes in your project.