posted by Don .
The drawing shows three particles far away from any other objects and located on a straight line. The masses of these particles are mA = 363 kg, mB = 517 kg, and mC = 154 kg. Find the magnitude and direction of the net gravitational force acting on each of the three particles (let the direction to the right be positive).
There is not enough information here. You need to be given or have enough information to find rAB, rBC, rAC, where rAB is the distance between mass A and mass B, etc.
The magnitude of the force acting on mass A is
-G*mA*mB/rAB^2 - G*mA*mC/rAC^2
apply this same model to find the force acting on mass B and mass C