It would be a huge player advantage...probably in the 10% range? There'd have to be a new strategy chart though. There are many possible combinations.
With perfect play,
- Edge seeing one dealer card: 3.37%
- Edge seeing two dealer cards: 17.20%
- Edge seeing three dealer cards: 37.88%.
The strategy chart is available for two cards. I'm not sure if Mr. Wizard has it in his book.
The optimal strategy chart for seeing 3 cards is pretty easy to figure out.
Not only is the two-dealer card strategy not in my book, I'm not sure it exists in writing anywhere. I checked "Beyond Counting," and it doesn't have one. It should be obvious most of the time anyway.
Check page 141 of Beyond Counting.
I saw A,4 offsuit.
Play A64 or higher.