The option in Spell Law doesn't take the idea of Arcane Spell Users into consideration.
In fact, the option in Spell Law only says to double the costs of Pures and Hybrid spell users, those of Semis and nons are left alone.
And I wouldn't class an Arcane Spell User as either a Pure or a Hybrid, at least not technically.
And since Arcane users should have a spell list cost that is more than that for Pures/Hybrids and less than that of Semis, a cost of 3/* is both our only option to fill that definition AND makes sense over all.