That's why I think there's more to this cooling circuit than just routing coolant to the heater core. The 2019 DT 1500 is supposed to route heated water to the cabin first, and to the transmission second.
When I look at the 3 way valve descriptions, this is what is posted: Category: Air Conditioning & Heat > Air Conditioner and Heater Plumbing; Cooling > Transmission Heater
Just because it's difficult to find replacement parts on these new trucks online doesn't mean the parts are no longer on the truck. Hell, I've had warranty issues where the dealers are having difficulty getting parts.