The gravitational potential energy of an isolated system of three particles, each of mass m, at the three corners of an equilateral of side l is

Gravitational potential energy of a pair of particles = $-\frac{{\mathrm{Gm}}^2}{\mathrm{l}}$

Since we have three pairs, therefore the total gravitational potential energy is $-\frac{3{\mathrm{Gm}}^2}{\mathrm{l}}$