Considering that they turned Earendil's ship with the Silmaril into a star, they already have it, so to speak. They would need all three to bring the Two Trees back to life, so having just one wouldn't do them much good.
Silly Addendum: Besides, Earendil needed the Silmaril. He used it as a kyber crystal and made the ultimate lightsaber so he could take Ancalagon down.
Hey, it's as good a theory as any!