Quote from the index of The Silmarillion.
Silvan Elves - Also called Woodland elves. They appear to have been in origin those Nandorin Elves who never passed west of the Misty Mountains, but remained in the Vale of Anduin and Greenwood the Great.
The Nandor were those elves who turned back from the great journey. Although Teleri in origin, they cannot be counted as part of the Eldar, as they never travelled to Valinor or saw the light of the two trees.
Thus if Amroth were descended from a union of mortal and one of these elves, it would not be a union of mortal and Eldar, just mortal and elf and so have nothing to do with the three great marriages mentioned.
Also don't forget that Legolas was himself a silvan elf from Mirkwood, and could therefore have recognised a very distant kinship. There, I think I've rambled enough, hope I made myself clear.
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