MOSfet thermal design: die-to-package always DRAIN?
Most of the heat is generated at the boundary layer between the source and the drain not actually at the drain as you suspect. The way they are made however, the drain presents a nice flat surface to transfer heat through in larger metal tab packages.

For smaller packages, you need to consider the thing as a thermal mass as a whole. All three pins play a part in heat extraction from inside the device. Source and drain are most important, but the gate also adds a thermal release path.



With some packages, like the SOT-223 shown above, the drain is a harder connection to the die itself and will give the most direct thermal conduction path. The others are bonded through wires and heat transfer is more through the package to the pin. As such the pins are cooling the package more than the die.. but it all counts.