Since a spawner will activate if a player is within 16 blocks of it (in a sphere, not a cube) there DOES seem to be an area at Y=14 (+/- a couple) that may activate all three spawners.
I think, you'll have to try it, that if you stand at -74, 14, 405 you will activate all three spawners.
If you de-activate the spawners in pairs and then go to that location and spiders appear then you are close enough for the one not deactivated.
Try this with all three possibilities to establish where you need to stand.
I've never done an elevator/drop for spiders so I don't know how one would be built.
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