I think you mean two out of three, given that only three are variable. The penny is fixed, as you say, lest the set size fall sharply from 99 to 19 (if there's

I think you mean two out of three, given that only three are variable. The penny is fixed, as you say, lest the set size fall sharply from 99 to 19 (if there's no smaller than a 5c piece) etc. at which point optimal coinage varies anyway.