Universe consists of billion of galaxies. There are four different shapes of galaxies which are commonly named as: Elliptical, Lenticular, spiral and irregular. Out of which spiral shaped galaxy is the most common. Spiral galaxies consists of small stars, Whereas the lenticular and the elliptical galaxies consists of largely composed matured stars.
Irregular galaxies have indeterminate shapes and is very small. Therefore, Galaxy have four different shapes which do not contain the shape of triangle. Hence, Triangular is not a shape of galaxy.