Gen 3 do require a dedicated circuit. I follow your logic about sub panels but it is not complete and incorrect to state it that way. A circuit in electrical design goes from a panel (main or sub) to a load device. Gen3 require individual circuit breakers and individual runs back to a panel (main or sub). Gen 2s can appear as one load on a single circuit breaker. Yes, technically the combination of the sub panel and G3’s can be considered a single load in the main panel, but from the gen 3 perspective, they need individual circuits:
From Tesla’s gen3 manual:
“Wall Connectors are allocated to individual branch circuits”
Page 25 of
https://www.tesla.com/sites/default...ng/Gen3_WallConnector_Installation_Manual.pdf
The keyword here is individual circuits. Also look at the next page where each gen 3, whether it has a sub panel or not, has dedicated 60a breakers and a dedicated run from that breaker.