Have you tried using the 3-in-1 as a soil drench rather than foliar? It's supposed to be a systemic and might work better in your case as a drench rather than sprayed on the leaves. Also I've found that rubbing alcohol on a cotton swab gets sooty mold off pretty good. I ended up trying it with success on my little potted gardenia after getting the mealies off. However you'd want to do that on an overcast day and with a large bush outside, it might be a big nuisance. But maybe working on it a little at a time until you get through most of it might suffice and it seems to work well.

It probably has white flies, which secondarily produce the soot from their byproducts. It takes repeated spraying to get rid of them. I have used an oil + insecticidal soap or one of the stronger sprayable insecticides. If you regularly treat, it makes a big difference. Hard to explain why one side only would be affected- is one in more shade?