so all items that provide armor class bonuses should be left outside?
No, they are talking about items that set your AC to a given number. Bracers of AC 2 didn't give you a -2 bonus to your armor class. They set your AC to a flat 2 (which was good in AD&D).
well, AD&D did not know about resistance and reduction, only immunity.
That's fine too. Feel free to convert an immunity to resistance or reduction or just leave it as an immunity, depending on how powerful said item is supposed to be.
all ability scores should be considered as a multiple of 3, right?
I don't understand the question.
Also, & I just thought of this,
EN World may already have 3.5e conversion of the items you are looking at converting.