Hello Benedict, 


A lot of the this depends on your environmental aspects, and can be scaled depending on your system specs's as API calls are sent through the MC Management service to the Deployment service.  So "automate assigning 3000 devices to their respective groups" all at once, may be slightly taxing on the MS/DS, but as there isn't any "limitation on the number of REST-API calls on a certain period" you can definitely accomplish your goal with the API. 


In later versions of MC I've heard that we may have a throttling feature that would help you quickly to spread out the workload for such requests and improve the speed for large API calls  Keep in mind that technically you can build your own throttling in the program to ensure their system is not overloaded but until then the current features of MobiControl should still be more than adequate to handle this.


Hope this helps,


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