Getting names right

[Andrew in Germany]

I have no head for names, other than getting the difference between a rose and an agave right. But having an eye for the difference in plants of the same genus escapes me completely. An example here:
All 3 are pups of the same age and off the same agave, with, to my eye, completely different personalities. All three treated the same and planted in the same bed outside at the same time.

The cool, plain Jane, the smallest of the three


The twisted brother


And the flaming narcissist with colourful teeth and terminal spines unlike any of its family.



How does one recognize that all 3 are the same agave genus? To my eye I’d bet 1 and 3 are of no relation other than being agaves.