What does a "3 h5" mean in reference to the axle of a motor?
I think it's a reference to a tolerance. Look at this ISO table. This States that the nominal diameter of 3mm is held to a tolerance of +-0.004mm.

@DarthPixel provided some great links identifying the term interference fit (or press fit as I've heard locally) as described here. Also, here is a better link providing examples of how the tolerance works and more legible tables to reference the correct tolerance. Keep in mind that the units in the tables is in nanometers.