Yes, TRIANGLES is a Scrabble word!

Letter values in Scrabble US: T 1, R 1, I 1, A 1, N 1, G 2, L 1, E 1, S 1.

Is it a valid word?

TRIANGLES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a three-sided polygon
something approximating the shape of a triangle
a small northern constellation near Perseus between Andromeda and Aries
any of various triangular drafting instruments used to draw straight lines at specified angles
a percussion instrument consisting of a metal bar bent in the shape of an open triangle

Not playable in Scrabble US: TRIANGULUM.