Is triangle a compound noun

NO!!! in modern English.

However, it is a combination of 'tri' and 'angle'.

Since 'tri' is NOT a Word in modern English, then it does not make 'triangle a compound Word. Do not confuse with 'try'.

Triangle is split into 'tri' and 'angle'. 'Tri' from Latin meaning 'three(3)'. Hence 'triangle' means 'three angles'.

Archaically this would be known as a 'Trigon'. 'Tri' ; three, and 'gon' from 'polygon' a 2-dimensional figure of any number of sides.