In my ipod (or in my mp3 collection, whichever), I want to clear the
"artist" field of any mp3 where I have less than 3 songs by that
artist. Is there any easy way to do this?

Hi there,
What you are trying to achieve could only be done if you could edit
the firmware of the iPod.

Dude man this is so easy its stupid,
while in itunes, right click the song you want to change.
from there select the info tab.
In this field you can chnage any information about the song you wish.
from track listing, to whomever composed it.
and thats it. click ok and you'll see your changes take effect.