I am instructed to enter three force values when using the force_element_edge command with only the material_velocity command in the initialization part of the input file during 3D analysis. Do these three values correspond to the values of the distributed forces in the x, y, and z directions?
Yes, you are correct.
However, if all that you want to do is apply a pressure, you cal also use
force_element_edge_normal which will apply a distributed force normal to the geometry entity that you are defining.
You can find examples inside the examples folder in the distribution.