(1) After the lever is balanced in the horizontal position, the distance from the fulcrum to the point of force action is the arm of force. Therefore, in this experiment, we should first adjust the lever to a balanced horizontal position to facilitate the measurement of the arm of force.

(2) Let the gravity of a hook code be G and the length of a cell on the lever be L, then according to the equilibrium condition, we can know that 4G×3L=nG×2L.

The solution is n=6 (pieces), so six identical hook codes should be hung at B;

(3) "Force× distance from fulcrum to force point = resistance× distance from fulcrum to resistance point" is obtained under the condition that the lever is balanced in horizontal position and the directions of force and resistance are vertical downward, that is, the direction of force or resistance is not changed during the experiment. Therefore, options A, C and D are wrong and option B is correct.

So the answer is: (1) arm of force; (2)6; ? (3)B。