Found a triple cavespider spawner on the server in a mineshaft and went to build a prototype of it in creative (calculated the positions of spawners so they match the server ones):


Green area=Area where mobs spawn (9x9)
Red area=Area where player needs to be for the spawner to be active (16 blocks from the spawner to each direction). This one is a bit misleading in images since it's actually sphere instead of cube, but the area doesn't change much.
Black area="Escape area", the area which mobs need to be out of for the spawner to be able to spawn more mobs.
So ideally if I want all three spawners working, I'd try to find a kill area outside the black borders (To get more mobs continually spawning) but inside all the red borders (To have the spawners active). And there happens to be one. Will probably build this contraption during the weekend
