How does a TB3 robot get the coordinates of others?

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This is not something that is done trivially with ROS. You need to fulfill additional requirements, that are more or less tricky:

1. same coordinate system: You need to somehow reference the robots in the same coordinate system. This could be the same map they localize in, a world frame of which you now the location of in the different maps or an indoor position system as it is done for UAVs.
2. communication between robots: this can be done using custom protocols or using one of the multi-robot approaches, e.g. multimaster_fkie.
3. navigation: obviously, your navigation packages need to handle those robots correctly as well. There have been some approaches, but I no of no specific open source one that is used more often.

For indoor positioning, the best are probably motion capture tools like OptiTrack or ViCon. But know that they will be WAY more expensive than your three TBs....