Three blocks are pushed along a rough surface by a force with magnitude P, as shown above. Fc is the magnitude of the contact force between blocks 2 and 3 and Ff, Fn, and Fg are the magnitudes of friction, normal, and gravitational forces on block 3, respectively. Please draw a FBD.

See the FBD attached

Explanation:

Since Ff, Fn, and Fg are the magnitudes of friction, normal, and gravitational forces on block 3, you can build the FBD (free bdy diagram) for block 3.

The FBD of the block 3 must include all the forces acting on the block 3.

The FBD is shown on the graph attached.

Observe this:

Fc is the contact force that block 2 exerts on block 3. Assuming block 2 is to the left of block 3, the force is toward the right.Ff is the friction force exerted by the ground on the block 3. Its direction is opposite to the movement. Assuming the block is moving to the right, the direction of Ff is to the left. Also, assuming the block is accelerating to the right, the magnitude of Ff is less than the magnitude of Fc, and the arrow that represents Ff is shorter than the arrow that represents Fc.Fn is the normal force on block 3 due to the contact with the ground. It points upward, as it opposes the gravitaional force. Assuming the block is not moving vertically, the magnitude of Fn is equal to the magnitude of Fg, and both arrows have the same length.Fg is the gravitational force: the pulling force of the Earth on the block. It is equal to the weight of the block.