Are you serious? That's impossible, I think. No mass can exert gravity on itself, just on other objects-it's fundamentally an attractive force. For a 3 kg mass, using just mechanics , any force that is exerted by this mass is exerted on the environment around the mass, not on the mass itself. A small correction would be that if you consider the first mass a collection of small masses that comprise the larger mass, then each small mass would exert a force on the other smaller masses. But I am beginning to diverge from the original problem. Anyway, if a second mass was nearby, the first mass would exert a grav. force on the second mass since that's considered part of the environment of the first mass. In turn, the second mass would exert an equal and opposite gravitational force on the first force. It's an action reaction pair of forces.
