Hi @katiebroyles,
This isn’t possible out of the box but there are a few ways to approach this.
Currently, the best option (albeit priciest option) would be custom code actions in Operations Hub Professional. The idea here is that workflows can now execute JavaScript which allows you to extend workflow functionalities. In this case, another user already shared the code needed. You can find it as point 9 in the The Definitive Use Case Library For Programmable Automation In Operations Hub.
Sticking with code, another user has created a PHP script, demo here, which he’s selling for $50/month or a one-time fee of $500 in this thread.
Another option: You can “hack” existing workflow rotation functionalities by nesting rotation or by working with an additional owner property as described here, but this gets convoluted very quickly, unfortunately: You would have to create 20 fake users to achieve your desired split, then rotate incoming records between those 20 users. Once rotated, you would re-assign records of users 1-9 to your actual first user, records 10-18 to your actual second user and 19-20 to your actual third user.
Lastly, while it might not be ideal, manual assignment is always an option. If Operations Hub Professional is too expensive, this could be a task completed once per day by a manager, with the obvious downside of potentially long sales response times.
Hope this helps!