I'd say that if the DAO holds 3 stable coins in equal value, it would be an equal percentage of each in order to ensure that there is deleveraged risk from each stable coin it holds.

Alternatively, if the distribution of the dao's stable coins was

40 A
20 B
40 C

You would see an equal measure taken from each stable, except in the situation where it is already HBD. Thanks for adding this as a technicality to the proposal and enriching it with your question.