The “21 + 3” blackjack side bet is based on examining the player’s two cards and the dealer’s up-card.
The point of attack I considered is to target flushes. Any strong imbalance in the suits favors the player.
This can be accomplished by a team of counters, each keeping track of one of the suits.
There are two reasons that APs will not target 21+3 with this system. The first is its complexity, the second is the low return.