is there a special way these 3 drives have to be hooked up.
whats in my system now is a #1 cd-rom #2 cd burner on one cable.
now i want to hook up a dvd burner. what do i do.

I can't see any reason for anything special , just put it on the primary cables secondary connecter and hope for the best , most dvd burners should have  buffering protection and I have never had any trouble with a cd writer on the primary cable up to now.

How many IDE's do you have? If 4 Just hook up and make it master.
If 2,
How many Hard drives?* If you only have 1, I would hook up the CD-rom up on the hard drive IDE as slave.* Then make the DVD Master on the second IDE.

Also if you have to, the DVD burner will do most if not all that your cd writer will.