they each drop only 1 particular key and the others do not drop that key, so you'd want to run one until you got 3 of a kind.
each lesser evil drops a body part, and the other lesser evils do not drop that part. for instance, each one is named appropriately, such as the part that izual drops is called "izual's insertbodypartnamehere," etc. unlike the keys, body parts will always drop the first time around, and LIKE the keys they will always have the same names, so there are only three, and how you place them in the cube etc. doesn't matter; there is no order that these or the keys have to be "done" in.
this doesn't change anything, but the names of the body parts may have the names of the prime evils instead, i can't remember.
so, since izual is the easiest you'd probably hit him first, so find out whichever one of the bosses drops the key to izual and keep running that boss until you get 3 of a kind. again, whether you have to cube keys in harrogath for the lesser evil portals to open i do not know.
btw, do you know someone named scott who i believe has two brothers? i'm reaching here, but wth.