We can trace it all the way back to the beginning. They are equally willing to throw their father to the winds, when he demands his humanity. And when each sees the opportunity to satisfy her feminine drive, each is willing to do anything — to poison or stab the other — to win the prize. Indeed, Regan and Goneril are hardly characters at all. Shakespeare sketches them as easily as he does the obsequious servant Oswald, imbuing them with all the motives, nuances, passions as would befit a tigress or a two-year-old.
Nor are they really different from each other. At the end of the play, only the Shakespeare scholars could tell you who claimed Edmund first, who stabbed whom, or who dies when. The reason is, of course, that it makes no difference to the play. They are interchangeable. They are bad sisters, bad wives, bad daughters. Compare them to the other evil force in the play, Edmund. Regan and Goneril serve essentially the same function as he does: to trick the father for their benefit, at the cost of his sanity and his life.
Edmund, however, offers us all the intricacy of a developed character. When we meet him, he reveals to us what it is that is bothering him. We even like him against our will. He is what Regan and Goneril should be if Shakespeare had wanted to make them come alive.
The third sister is no more real than her elders. She is good, pure and simple. Love, and be silent. Then, when the evil ones are tossing their father back and forth, propelling the story forward, Cordelia is in France, as absent as ideals are in that murky middle of the play. When she returns, she is the heroine, the friend, and the martyr all in one.
She is an angel on the stage, and an fallen angel in death. In short, she is entirely unambiguous, totally single-minded, and, as characters go, uninteresting. Edgar, her counterpart in goodness, has so much personality that he is forced to split himself in two.
