The gravitational force acting between each couple of masses
F = \(\frac{G M M}{a^{2}}=\frac{G M^{2}}{a^{2}}\)
Each internal angle of equilateral triangle is of 60°. Therefore the resultant of forces acting on each mass will be directed towards the centre O of the circle and the same will the role of centripetal force for circular motion of the whole triangle.
Centripetal force = Resultant force