The license for the single vendor version allows 3 storefronts, correct? There is a storefront product available. Is this needed only if there are already three storefronts?
If you want to have 3 store-fronts on one CS-Cart installation, you should buy one lifetime license:
CS-Cart licenses :: CS-Cart License -
and 2 store-front licenses:
CS-Cart licenses :: CS-Cart Ultimate License -
Only one store-front is allowed by the lifetime license