There are several classes in Baldur's Gate 3 that don't make a subclass choice until Level 3. But when these classes do end up getting their subclass, it's a huge leap in power – especially for Fighters who go with the Battle Master subclass. This subclass unlocks a variety of Battle Maneuvers for the Fighter to use, which use a unique resource and give the Fighter more control over melee combat.
Baldur's Gate 3: 5 Best Battle Master Fighter Maneuvers, Ranked
Battle Master Fighters in BG3 get access to new abilities that use Superiority Die, but some of these Battle Maneuvers aren't worth picking up.