Need a formula to calculate stats for multiple classes

Hi-

This is a bit complicated, but hopefully I can explain my problem clearly:

I've generated a raw numerical output in a different program (ArcGIS, specifically), and I need to use it for some calculations. I've input it into excel, and it's organized as follows (see attached jpeg for a preview):

First column: "Zone" - these are the zones that I extracted the stats from, numbered about 0-580. There are multiple subsequent values for each zone, so the same zone value appears more than once.
Second column: "ID" - There is an individual ID for each of the values, ~0-2500 or so (lots of data).
Third column: "Class Value" - There are six possible value classes, numbered 0-5. Some of the zones have all six classes, and thus take up six rows, while some only have 5 of the 6, or 4/6, etc.
Fourth column: "Count" - The actual raw values that I want to work with.

What I want to do: For each zone, I want to get the percentage of Class Value "3" as compared to all the classes combined. For example, for the first Zone value (0), I'd add all the class counts together (650+27885+1341+5+47+1), and divide by the class 3 value (5), and multiply by 100 to get the percentage of that class within that zone. I want to do this for each of the 580 zones. The issue is, not every zone has all six classes present, and so if i were to create the formula from the first set of values (the red ones), and then drag it down to the bottom, the stats would be shifted incorrectly at the end, but I have way too many values to do it manually. I hope that makes sense.

So what I'm thinking is to somehow make the function work by grouping the zone values together? But I'm just not sure how to go about it. Any advice would be extremely appreciated. Thanks!

Cheers,
Emma