Sorry if this isn't the right place to ask, but it seemed like the most relevant stackexchange.
I want to print a connection from a motor to an omniwheel, the axle of the wheel is listed as 8mm, but the one of the motor only say "3h5" which isn't really helpful for me, and neither was Google.
Side-question: Would pla be sturdy enough or do I need to use abs?
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.