Didn't we already do this? If you have 20 years of data, why not just take it from the data?

There remain only six outcomes:

ABC

ACB

BAC

BCA

CAB

CBA

Event(A>B)= U(ABC,ACB,CAB)

Event(B>C)= U(ABC,BAC,BCA)

Event(A>C) = U(ABC,ACB,BAC)

Event(A>Both) = U(ABC,ACB)

Off the top of this third rumination, I'd still say you need more information.