I know she has 6 books...and they tend to be grouped in 3s...The Kitchen Witch, My Favorite Witch, and The Scot, the Witch, and the Wardrobe.  And then Sex and the Psychic Witch, Gone with the Witch, and Never Been Witched.

Do I need to read the first set of 3 to understand the last set of 3?

i've read the last 3, and not the first 3, and wasn't confused.

the character from 'The Scot, the Witch, and the Wardrobe' is related to the girls in the 2nd trilogy, and briefly turns up in it, but i didn't feel at all like i was missing anything from not having read those first (though i intend to get to them eventually).

I've read all of them as library books and am slowly adding them into my permanent collection. I would read the individual trilogies in order, but you don't need to read one trilogy to be up to speed on the other. It's late here, so hopefully that makes sense.

It kind of helps because you meet the triplets in the first series.  They're a good read.