I think the number of list variables is a limitation of AA's data store backend. So there's really no way to get more, even if you're willing to pay more.
Are all 3 list variables already in use? If there's one spare one, you could do the following: track it with another eVar, where that other eVar indicates the purpose of the list.
E.g.
eVarXX = "product catalog";
listX = "product A,product B,product C";
and
eVarXX = "internal promo banners";
listX = "banner 1,banner 2, banner 3";
Then when reporting listX, you'd always want to report it with eVarXX so you know what values to get back.