Why is it difficult? It certainly adds a layer of complexity but if you define the three instances ahead of time and send appropriate keys to each one based on their specs, then it should work pretty well.

At the simplest, you could put your in-game attack macro on your favorite key(s) and then just use broadcasting software to /assist and then pure broadcast those attack keys among the three.

At the more complex end of things, you can use something like ISBoxer which will let you define complex, spec-based macros and multi-step systems that will dynamically manage your team potentially much more effectively.

Question for you: what multiboxing tool(s) are you using today? Need free or have some budget to spend?