It is far more likely that the three basins were chosen for the slight imperfections in them that influence the direction of the rotation of the water. And the middle one, "on the equator" was probably the hardest one to "tune up" to get zero rotation.
It is quite possible they all have some fins inside the drain tubes which direct the water.
If you have ever played with a sink full of water and started the rotation when the stopper is pulled, you may have noticed that it does not take much of a start to get it to go one way or the other. Once it starts going one way, the effect is amplified by the velocity of the water going down the drain so it goes faster and faster as it drains. Try it in your bathroom or kitchen. Drain some sitting water a few times to see which way it wants to go. Then, do a run and give it a bit of a nudge in the other direction and watch it go that way just as fast.
Oh, and if you want to see the effect more easily, use a larger diameter drain. The larger the drain, the greater the effect because there will be more amplification.