On a two-card total of 3, the Banker draws when the Player stands on 6 or 7, and draws against 9 out of 10 Player third-card values (0, 1, 2, 3, 4, 5, 6, 7, and 9)—standing only when the Player's third card is an 8.
Why does the Banker stand on 3 against a Player third-card 8, yet draw against a Player third-card 9? Because the Player only draws on two-card totals of 0 through 5. When the Player draws an 8, their new three-card total becomes 8 (from 0+8), 9 (from 1+8), 0 (from 2+8), 1 (from 3+8), 2 (from 4+8), or 3 (from 5+8). In four of those six outcomes (0, 1, 2, and 3), the Banker's existing 3 already beats or ties the Player without risking a 7, 8, or 9 that would wrap the Banker down to 0, 1, or 2.
Dealers memorize Banker 3 by a single exception: 'Three stands on eight.' If the Player stands on 6 or 7, Banker 3 draws. If the Player draws any card other than an 8 (including a 9 or a 0-value face card), Banker 3 draws.