If drive A has a data density (bits per square inch) that is four times greater than drive B, then we could expect that A would have twice as many tracks per inch as B, and twice as many bits (or sectors) per track.
The data rate is …
(sectors / track) x (bytes / sector) x (rotations per second)
I expect that your WD5000AAKS-00V1A0 drive would have a single disc with a density of 500GB per platter.
Without knowing anything about your WD5000AAKS-75YGA0 drive, we would expect that its data density would be …
500 x (79.6 / 131.4)^2 = 183GB per platter.
So this would suggest that it has 3 platters.
Similarly for the 320GB drive, its data density is …
500 x (105.9 / 131.4)^2 = 325GB per platter.
So this would make it a single platter model.
BTW, I’m assuming that all drives rotate at the same speed, ie 7200RPM.