What you're looking for is a floor-leveling product. W/O recommending a specific Brand, visit the Flooring Dept. at your local Home Center for products. If you can, in advance, measure the amout of 'rise' you'll need to level the floor. Some products will 'allow' a certain thickness. If you need 3" and the product says there's a MAX of 2", you don't want to go any higher. In cases where you need to go higher, consider Planed Sleepers installed UNDER the sub-floor. (They're not really shims ...)
My best to ya and hope this helps.
Jay J -Moderator