Actually, the demand factor is not determined per phase, but is rather determined for the whole panel and then applied per phase. I haven't taken the time to review my previous comments to see whether I let you astray; I'm sorry if I did.
I have Excel files from two different firms. Although the formulas they are using are slightly different, they're actually the same thing. As in the calculate the exact same way.
Example 1:
A = if(A+B+C <= 10, A, A / (A+B+C) * 10 + 0.5 * A - A / (A+B+C) * 10)
Example 2:
A = if(A+B+C <= 10, A, (10+(A+B+C-10) * 0.5) / (A+B+C) * A)
The second example makes most sense to my brain. The expression (10+(A+B+C-10) * 0.5) / (A+B+C) is determining the demand factor that is applied to the receptacle loads on the panel (this would be the percentage that the current Revit panel schedules report). That demand factor is applied to each phase. The first example is somehow the same formula rearranged because it calculates exactly the same, but I haven't taken the time to wrap my head around the logic behind how it was developed.
So in your example of 10 kVA of receptacle load per phase for a total of 30 kVA on the panel: total receptacle demand on the panel would be 20 kVA, divided to 6.67 kVA per phase. This jives with how I understand the code. At first glance, it may not makes sense because changing one phase can affect the others. For example, here's 10 kVA per phase:
And here is phase C increased to 20 kVA:
What may not make sense at first glance is that increasing load on phase C actually decreased demand load on phases A and B. But it does make sense if you think of it as first determining the receptacle demand load for the panel, then applying that demand load to each phase.
The same would be the case in your example of multiple dryers on different phases. Determine the demand factor based on the total number of dryers, then apply that demand factor to each phase. So, to use your example, total connected load for A, B, C would be 4000, 4000, 4000, the total number of dryers on the equipment is 6, so the demand factor applied to each phase is 75%, and demand for A, B, C would be 3000, 3000, 3000.
If you delete the dryer on circuit 6 in your example, total connected would be 3000, 4000, 3000, demand factor would be 85%, and demand would be 2550, 3400, 2550.
Hopefully that makes sense.