Well reports are that the Medallion Fund is capped at $10B with a 66% annual return before fees and a 39% annual return after fees, so the other funds would need to underperform by an average of at least $3.9B per year. If we include the fees in the performance (it's not clear to me that we should, since it's just an accounting measure that RenTec claims to pay itself) then it's at least $6.6B.

Wikipedia says RenTec AUM is $110B. $10B for the Medallion Fund leaves $100B for the other two. So they would need to underperform by either >3.9% or >6.6%. I have had trouble finding exact numbers for those funds, so I can't determine if that's really the case or not.

However, this assumes RenTec is unable to beat the market at all. Maybe they really can beat it, but by only 2-3% per year. They can still funnel that outperformance into the smaller fund to turn it into 66%/39%.