This question is not well enough explained. Are the masses in empty space and attracted only by the gravitational attraction of the other three masses? Or are they sitting on the ground somewhere? In the latter case, if they don't move, the net force on each mass is zero. In the former case, use Newton's universal law of gravity

F = G M1 M2/R^2

and add up the forces due to the other three masses, treating them as vectors. There will be a VERY small net force of attraction towards the center of the square. R = 1.5 meters for the adjacent two masses and 1.5 sqrt2 meters for the mass located diagonally across. For the two adjacent masses, the vector sum is along the diagonal.