Yahoo Fantasy Sports doesn't support trades that involve more than two teams in a single transaction.
See Also
How to Recover from a Big Trading Loss? - VRD NationHere Is a Look at How Long to Hold on to Stock in Day TradingThe Anatomy of Trading BreakoutsVetoes by President Donald J. TrumpIf a three-team deal is negotiated, you can still process it by submitting the trades individually, but understand that teams are under no obligation to accept trade offers.
Only proceed if you're certain the third party will agree to the additional trades.
