Alternative to simultaneous sequential call queues?

A customer dials a direct number, and I have 3 groups of call queues (Sales, Customer Service, Fallbacks) that I had hoped to overflow to the next one after x amount of rings. However, Overflow Groups don't allow our preferred simultaneous calling on the primary call queue. Does anyone know of any other way to emulate a waterfall-like call queue system that does simultaneous calls for each member in each round/group?