∴ If the car is in a state of balance in the horizontal direction, the thrust and friction in the horizontal direction are the balance forces;

∴ Friction force f = f = 2n.

When the horizontal thrust increases from 2N to 3N, the roughness of the contact surface and the pressure of the trolley on the ground remain unchanged.

∴ The friction on the car will remain constant at 2n.

The bookcase door at home is not tightly closed, so a piece of paper is folded several times and sandwiched between the cupboard door and the door frame, which increases the pressure and friction between the cupboard door and the door frame, so the cupboard door is tightly closed.

So, the answer is: 2; Constant; Stress.