two masses attract each other with a force f.if a third mass is placed at the midpoint of the line joining them ,what will be the change in this force?

If we just simple take the force acting on the body, then the force pulling those two bodies will increase, but the force applied on each other will remain same.

The pulling force increase due to the superposition principle as the force increase due the force superimposed by the force exerted due to the new body.