Wouldn't you divide by 1000, not multiply, as you are talking about converting to ? If you have 3000 moles per decimeter cubed then surely there would be 3 moles per centimeter cubed, not 3,000,000. I expect this is what was meant, but it could easily be misinterpreted.
As a side note you should really know how it works in case you forget what to do. 10cm=1dm. =1000. =1. Therefore .