The FCC rules require HD1 to have audio quality comparable to analog FM. But they've never been more specific about what they mean by that, nor has (as far as I know) there ever been any enforcement of that rule.So, in theory, HD-3 could be assigned greater fidelity than the analog station's required HD-1 (those who are more experienced than me can correct this if I am off... I have not done real engineering for decades).
But aside from that, stations are free to split up the HD bitrate pie any way they want.