Phy~Friction

You are right. The wording of the question have some problem to use "unless "(除非). It should be replaced to "until "(直至) or the sentence should replaced to "A box doesn't move in constant velocity unless the pulling force is 3N" or may change to "unless the pulling force is larger than 3N". It mean that the box will start to move when the pulling force just larger than 3N.
And you said the box is not be able to move is not correct, when net force=0, it may move in constant velocity.

2007-04-14 00:43:19 補充：
If the wording of the question doesn't change, it mean that the box will move only if the pulling force is 3N but not larger than 3N. It is totally not reasonable.

The question gives you that the limiting friction is 3 N. So, when the box starts to move, the acting force must be greater than 3N.
In the actual fact, the question doesn't give you how large the force is, it only says that the box moves and asks for the frictional force.
Thus no matter what the force acting on the box is, as long as it starts to move, the friction must be at its max value and is equal to 3N.