This is the single most famous exception in the entire baccarat tableau: Banker holds 3, Player draws an 8 as their third card, and the Banker MUST STAND.
When the Player draws an 8 onto an initial total of 2, 3, 4, or 5, the Player's modulo-10 total wraps around to 0, 1, 2, or 3. Standing on 3 lets the Banker beat 0, 1, and 2 outright and tie 3 without risking a high card that wraps Banker's 3 down to 0, 1, or 2.
For Banker 3 facing Player's third card 8 (8), combining 8 with the 6 legal Player starts (0..5) yields the specific Player 3-card sequence [0+8→8; 1+8→9; 2+8→0; 3+8→1; 4+8→2; 5+8→3], against which Banker 3 (10+3) tallies 3 standing wins, 1 pushes, and 2 deficits.