Level 3 subclass
Way of the Open Hand is a monk subclass at level 3. Its Flurry variants are Topple, Stagger, and Push.
Flurry of Blows is a bonus action, spends ki, and makes two unarmed strikes. It does not trigger Extra Attack.
Stagger and legendary reactions
Stagger is the variant that talks about reactions. The Honour note beside it is narrow, so read it before you plan a boss around it. Stagger does not reliably shut down Honour Mode legendary actions: many use a reaction label without spending the Reaction resource, so use another control condition if that boss action must be stopped.
Stagger blocks reactions for 1 turn.
The Stagger page notes that many Honour Mode legendary actions are reactions that do not spend the Reaction resource.
Tavern Brawler and the unarmed die
Feats arrive at class levels 4, 8, and 12. Fighter adds one at class level 6. Rogue adds one at class level 10. A multiclass that leaves a class on level 3 skips that class's first feat.
Tavern Brawler gives +1 Strength or Constitution, to a maximum of 20. Unarmed, improvised, and thrown attacks add the Strength modifier a second time to the attack roll and the damage. If the unarmed attack uses Dexterity, the extra addition is still Strength. Dexterity is not added a second time.
Martial Arts: Deft Strikes replaces the 1 base damage of an unarmed strike with 1d4 at monk level 1, 1d6 at monk level 3, and 1d8 at monk level 9.
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