The three steep-sided mountain peaks that stood out above the gorge of Helm's Deep. They were the northernmost of the White Mountains, and so they also marked the southern edge of the wide Gap of Rohan. The name 'Thrihyrne' is from the Old English for 'three-horned' or 'three-cornered'.
Notes
1
There was a real Old English word Þrīhyrned meaning 'three-cornered' or 'triangular', but this original meaning does not seem to apply directly to Thrihyrne in the White Mountains. We're told that the mountain had three separate peaks (actually described as 'three jagged horns' in The Two Towers III 7, Helm's Deep) so in this particular context the name was apparently intended to literally mean 'three-horned'.