@Kilian KYes, I have attached an updated version of your model to this comment as well as an example model that shows it more clearly. Let me know if this is what you're hoping the behavior to be. I used an Event-Triggered Source to listen to when the Queue has 3 or more items (shown in original reply). If it does, the activity creates a token that moves into a Custom Code activity that stops the Multiprocessor.

Then the token moves to a Wait for Event activity that listens for the content of the queue to drop below 3.

When this occurs, the token moves to a Custom Code activity that resumes the Multiprocessor.

Example Model: stop-processor-example.fsm