In Old English, the adjective þryscyte 'triangular' (Icelandic þriskeyta n. 'triangle') was used to define triangular shapes. Additionally, gar~gara suggests triangularity but it was only used for land masses and it is not always translated as a triangular land mass.
Here is a dictionary entry for þryscyte from An Anglo-Saxon Dictionary by Bosworth and Toller (1898):
þri-scíte; adj. Triangular, three-cornered :-- Ispania land is þryscýte Hispania trigona est, Ors. 1, 1; Swt. 24, 1. Sicilia is ðryscýte Sicilia tria habet promontoria, Swt. 28, 2. On ðone þryscýtan crundel, Cod. Dip. Kmbl. v. 374, 26. [Cf. Icel. þrí-skeyta a triangle.]
Here is an excerpt from the book Grammaticalization at Work: Studies of Long-term Developments in English (edited by Matti Rissanen, Merja Kytö, Kirsi Heikkonen) where you can see the translation of both Old English words:
Note: You can see the original text in archive.org also.
You can read much more details about geometric terms in Old English in the book Folk-taxonomies in Early English (By Earl R. Anderson) which also includes the following explanations:
Although Anglo-Saxon and medieval societies were "carpentered worlds", Old English had basic terms only for curvilinearity (hring and beag), and until the fourteenth century Middle English had only circul~circle and ring.
Like other Indo-European languages, Old English has no taxonym for 'geometric shape'; OE hiw (Gothic hiwi) usually means 'appearance'.
Because of the paucity of geometric terms in Old English, buildings in poetry may be described in terms of their height and magnitude, but rarely in terms of shape.