I think your reasoning is right. A question: how much shelf space does each brand have at retail? If A has 3x the shelf space as either B or C, then your option (a) may well accurately reflect the market. If A, B and C have equal shelf space, then your option (a) may well bias results in favor of the Brand A portfolio. In this case your option (b) might work. In addition, an option (c) might work, too: allow each brand in your choice sets in proportion to their retail shelf space. So if A, B and C have share in a ratio 3:2:1 maybe you have six products: A1, A2, A3, B1, B2, C.