The windings in a 3 phase motor, when activated by a 3 phase supply produce a rotating magnetic field in the rotor area of the motor. Swapping phase A with phase B re-orders the fluxes so that the flux rotates in the opposite direction. Swapping B with C does exactly the same thing as does swapping A with C. Think of it like a triangle with corners called A, B and C. If you swap any two corners and follow the points A, B and C you'll go in anopposite direction. Swap two more corners and you're back to the original rotation.

This is what it looks like. The black arrow is the flux produced by the three phase windings: -



Clearly, if yellow phase were swapped with blue phase the rotation would be opposite.

How to change rotation direction of 3-phase electrical machines?
Recently I have been looking up James Joule's experiment regarding the mechanical equivalent of heat.  After viewing some drawings of the apparatus, I assumed that the lines holding the weights would have to be attached to the drum.  They would then be allowed to fall completely through a certain distance until the lines were at full length and the mechanical motion of the fluid would have to be allowed to continue to flow, causing the weights to be pulled up a bit and drop again, oscillating up and down until the motion stopped and all of the mechanical energy from the weights would be transferred into the fluid.

However, upon watching some videos, (this video is about 14 minutes long; but, if you advance to about 5 min 30 sec, it will show the experiment in question.) I noticed that the weights were allowed to be stopped by a level surface (an external force) instead of doing what I have described above.  To me, this means the weights were still in motion and, therefore, still had KE that was not transferred into the fluid.  

Of course, the weights are moving very slowly, much more slowly than if they had been allowed to free-fall through the distance; so, their kinetic energy is very small compared to the potential energy they lost, meaning the vast majority of the potential energy was transferred into the fluid and into other, small energy losses.

Should I assume the speeds of the weights were small enough to ignore the KE in the weights?

The assumption seems to be that the potential energy lost by the falling weights was all transferred into the fluid.  However, in the experiments, the lines have to be wound back up and the weights dropped several times in order to make a measurable difference in temperature.  Each time the weights fall, they are stopped by an external force, meaning they have kinetic energy that is not transferred into the fluid after falling through a measured distance.  It seems to me that some of the potential energy from the weights is transferred into the fluid; but, some of it is transferred into the kinetic energy of the weights falling.
