Borderlands 3 has 3 Circles of Slaughter Locations. For the Slaughterhouse 3 trophy / achievement you must finish all 5 rounds in each of the circles of slaughter.

Luckily, none of them are missable. You can still access them all after the story. The main story missions will bring you to all areas from where to access the circles of slaughter. Just if you don't know about them they can easily be overlooked.

Circles of Slaughter are a sort of horde mode where you fight waves of enemies that get stronger with each round. There are always 5 rounds in total to complete the Circle. After stepping into the Circle of Slaughter, accept the quest from the NPC and finish all 5 rounds to complete the quest. This is repeatable.