Glamdring

Glamdring was the magical and powerful sword of the Noldor King Turgon. Glamdring was arguably the best blade recorded being discovered in the Third Age. Glamdring was hated by the Goblins more than its sister sword Orcrist. Glamdring glowed Blue when orcs were near, but it also had the ability to turn into a holy white when facing dark servants of Morgoth (like the Balrog of Moria). Eventually the sword was found by Gandalf and Thorin's company after the death of William, Tom, and Bert. How exactly the three mystical items of Gondolin came to be in an area of the Ettenmoores, is a mystery.

Speculation
It can be hypothesized from Elrond's words, that the three trolls plundered other plunderers. A generalized consensus on how Glamdring appeared beyond the borders of Beleriand can be seen here: After the fall of Gondolin, Celebrimbor takes the artifacts of Gondolin with him when he flees. After the destruction of Ost-in-edhil in the Second Age, the armies of Sauron confiscate the weapons. The forefathers of the three trolls then attained the artifacts by taking them from others. Eventually the three trolls get them passed to them during the Third Age after they come to adulthood.