(idiomatic) just right, most suitable / bring me...

a right-angled triangle

acute triangle (triangle all of the angles of which are acute)

obtuse triangle

a tridecagon

(uranography) Winter Triangle - a bright asterism of the northern winter sky. Its corners are the stars Betelgeuse, Procyon, and Sirius.

(uranography) Summer Triangle - a bright asterism of the northern summer sky. Its corners are the stars Altair, Deneb, and Vega.