The weight of the ball is the y-component. There are two equations for weight. Well, in the weight equation, the mass is for some object held close to the surface. It is this mass which cancels. The radius is the distance between the two masses.
G stands for the constant for the Universal Law of Gravity.
Little g is not a true constant. In doing this you must first compute the forces from three of the spheres acting on a single sphere. Then draw your vector forces. Recall how to add vectors. The resultant force will also be directed along the diagonal.