And you're wrong...
By knowing the ball isn't under the third cup that changes everything. there were 33% from each cup. You have 33 for A / 33 for B and 33 for C / If you choose B you have 33 and still have 66 % it is on A or C.... BUT you know that it isn't in A so the 66% are all on C !
Look at movies "blackjack 21" or read the Monty hall Problem / Paradox
That's why she specifically ask to let a player draw the ball before... to know where the ball is not.
No, you're missing the point of Monty Hall, it's about information beyond the obvious "now that A is revealed as white we know black wasn't in A".
In Monty Hall, you pick one of A/B/C to check, then the host reveals one of the others to show that the prize/black-ball isn't there, and you're asked if you want to change your choice.
And if you chose B it's not just that there's 66% it was in A or C so it collapses to 66% in C, because it was also 66% in A or B and so collapses to 66% in B...
The thing is, when the host reveals what's behind A, they don't make the same choice you do (randomly between A, B, or C), they have two pieces or information, two constraints, on their choice, that don't exist for the other team in B.E.S.T:
1. They know, and will not show, where the prize (black ball) actually is. They know where it is, and won't chose it. I mean, can you imagine they show the black ball is under A, and then ask you if you want to stick to you choice that the black ball is in B or instead switch to C?
2. They know, and will not show, the option you chose. You already announced you picked B, and so they won't pick B. I mean, can you image they show B has a white ball, then ask you if you want to stick to B or switch to A or C?
And this gives you information, beyond just knowing that A held a white ball. Because when you chose B, you knew that there was 33% chance for each option, so 33% chance B has the black ball.
In the case where B indeed held the black ball, the host will randomly reveal either A or C but not B, and in this case (33% of the options) you should absolutely not switch.
But in the case where B held a white ball (66% of the cases) the host will not randomly choose between the A and C. They will always reveal the one that holds the white ball, because they know the other is black. So if you picked B when the black is in C (33% of the case, or 50% of the cases where you picked B and B is white) they will only show you A always, but if you picked B when black is in A (same percentages) they will always only show you C and never show you A. So in these two cases, you should always switch.
The thing that makes the difference (why 66% for AC matters but 66% for AB doesn't matter, when you picked B and A is revealed) is that the host knows where the black ball is, and what you picked. They didn't pick in random. If you picked B and they showed A then it's always either because the black is in B (33% of the cases) or the black is in C (66% of the cases, because if it was in A then they would have shown C instead, not A).
In B.E.S.T this doesn't happen in the story. If you picked B, the other team could have (in story) still picked B. And if the black ball was in A, they could have still picked A. They don't have the extra information, so that extra information doesn't put any constraint on their choices, so you don't get any part of that extra information from their choice.
Now, story-wise, since the episode did finish very little after this, you don't necessarily need to re-do the whole section. It's lucky that Ruby came up with the idea. You can just add a scene early in the next episode where Ruby tries to explain the logic, and gets an explanation and a joke that this is why Minus should leave the thinking to Cortex... ;-)
