A 100 N horizontal force is applied to a 5.0 kg box causing the box to move at a constant speed of 3.0 m/s. What is the value of the friction force on the box?

The friction force must be acting to oppose the 100 N applied force, so it should be - 100 N.

The reason for this is that the box is moving at a constant speed, with no acceleration. For there to be no acceleration, there must also be zero net force.