Question. Do I need to load the Cardano3 packages at each Mathematica session?  Or is it enough to load them the very first time only?

Answer. You must evaluate

Needs["Cardano3`ComplexGraphics`"]
Needs["Cardano3`ComplexRoutines`"]

once at each Mathematica session where you will be using Cardano3 functions.  (And be sure to evaluate those expressions before you use any of the functions in the Cardano3 packages!)

If you have done so in one notebook at a given Mathematica session, you do not have to do so in another notebook in the same Mathematica session.

The explanation for this is that, once you start Mathematica and begin evaluating any Mathematica expressions, Mathematica then “memorizes” all definitions you’ve made—and all definitions in packages you loaded by means of Needs statements—after that until you close Mathematica.  Then Mathematica simply forgets all the definitions it had remembered from that session.

Mathematica also forgets everything it learned at a session if you use the Quit Kernel command on the Kernel menu.  And of course it forgets any definition you made at a session if later in the session you use Clear to tell it to forget that definition.