I think they should go for the expanded storage route. Adding a third retainer which would also have access to the markets would be a disaster for the markets.

Having two retainers and 3 markets means you must choose 2 markets to sell stuff on, forgoing the last one. This forces you to think about what you're selling and where, and allows strategies like selling the same item but in a different market in order to coerce people who happen to be shopping in that area to buy your item instead of someone else's equally priced item in another market, by being cheaper because no tax has to be paid. (and yes I said equally priced, no reason to undercut when using this strategy)

Adding a 3rd one would take this away for a part, plus you'd get 50% more items on an already flooded market (or rather: a market with lots of supply but little demand), which would turn a sputtering market into a drowned one.