As long as you don't burn the sugar, there is no problem.
Sorry to hear about your losses. You mentioned treating with acid for varroa, but did you treat with menthol for tracheal? I know there are a lot of folks that don't have problems with tracheal mites, and there are tracheal resistant queens on the market, but I still believe tracheal mites account for a large portion of winter kills. Since they are not visable to the human eye, I believe a lot of folks just figure "out of sight out of mind". Did you inspect the bottom boards for dead varroa? This will give you a good indication if your varroa treatment worked, or if they where the cause. You need to do your best to try and identify why they died so you don't go down the same path next time. Don't assume starvation was the root cause, because I doubt all 3 hives would starve and still have stores left. Although they ulitmately died most likely by starvation because their numbers dwindled to the point of not being able to keep warm and move to additional stores, this is the secondary cause. Just like a lot of people die from pneumonia it is not the root cause. It is that their bodies have been run down by something else such cancer.