Documentation says that you can network two or more Norstar system together. It seems like a pretty cool challenge, too!
As for the maximum number of MICS that can be networked together, it is hard to answer. For example, using a coordinated dialing plan, you could easily have 7 MICS networked together: DNs in first system would begin by 2, DNs in second system would begin by 3, …, DNs in seventh system would begin by 8. (I left out DNs beginning with 1, 9 and 0 as these digits are usually taken for the Call Park prefix, the external access code and the direct-dial digit.)
However, sooner or later, I guess there would be some issues. For instance, depending on how the public trunks are distributed, some calls might not be routed because private trunks are busy.
In your case, there would be only 3 systems. I'm sure it can be done. The hardest part will probably be to plan everything carefully: number of public trunks and in which systems, number of private trunks between the systems and numbering plan.