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Yes, this can be achieved by using robot groups capability which is available in version 220.127.116.11 and above. It lets you to combine robots to speed up test execution.
In test plan form, there is a field called “Robot group” which you can choose the robot/robot group to be used to execute the process in your test plan. “All Available Robots:” is the default selection. This means all available robots (in your case, all the 3 online rTesters) will run the Test cases of the selected process in parallel. But you can also create a specific robot group and choose it. This means that only the available robots associated with the selected robot group will run the Test cases of the selected process in parallel.
To learn how to manage robot groups, see this article.