The sedation is amplified through each level, remember? If their sedation was multiplied many times in the 3rd level, they probably wouldn't be able to feel much coming through from the first or second level.
There is a difference between apparent weightlessness and actual weightlessness. In the first dream, they are in free-fall, and therefore are experiencing apparent weightlessness. This transfers down to the second dream as actual weightlessness. The actual weightlessness felt in the second dream does not have any effect on the third dream.
Or if you don't buy that explanation here is another: Would the movie really have been any better if the entire snow action sequence was in zero-g? That would have been silly/impossible to film.
1st law of physics - INERTIA. In the 1st dream level. They were in free fall. Hence, there is a pull of gravity - there is an external force acting on their bodies which has an effect in their dream world bodies. In this case, it made them float in the 2nd dream level. So it doesn't have an effect on the 3rd level because there is no external force acting on their bodies in the 2nd level. It's as if they're just sitting down relaxed.