Along with the 3-passage torque converter, two passages are useful for transmission oil (ATF) circulation from the torque converter and for clutch cooling, when the third passage is applied independently to control the lock and unlock with the clutch. This is a function past what a simple fluid coupling gives, https://torque-converter-clutch19630.bleepblogs.com/34445070/not-known-details-about-torque-converter-clutch