In our dealing school, we have new blackjack dealers look over this list of three card combinations to 21 and commit it to memory. Soon, card tallying will be second nature. For instance, if you remember 6-7-8 is 21, then you’ll know that 6-7-9 is too many. You don’t have to know what 6+7+9 totals, just that it is too many.
Note: when counting, think of aces as having a value of 1 until you get to 11, 10, 9, 8, and (if the dealer doesn’t hit soft 17) 7. When you get to the above totals, then the hands become 21, 20, 19, 18, and 17 respectively.