Fortunately, I have read many, many times. On this very Forum. That ANYONE can fix a bike. So you should have no problem fixing the issue even if the guy in the shop can't. I will check back in fifteen minutes. I'm sure it will be perfect.
By the way to answer your question. Well, not to answer your question as I can't find my calculator. But, 8x3 does not = 24 when you are talking about ratios on a bike. Seems like it is around 17-18 different gears. But I need to do some gear inch calculations and chops are on the grill. They come first.

P.S. It is Sora, not the most precise stuff ever, tho it should work better than what he has. My bet would be the shifter is not setup to trim properly, or, the angle/height of the der is set up poorly.