D90 help !
If I understand correctly you are using flashes that are not built to work with iTTL. If your SB-600 is on your camera, perhaps it is sending a preflash. That is causing the other flashes to go off. Change your flash to skip sending the preflash. I am currently using two SB-600, an old flash that was my father in law's, and a fourth flash that is recommend by strobist. I need four flashes because I have to light up a green screen. I'm using ISO 400, 1/200 s, and usually f/7.1 or thereabouts.
Also try using manual mode.
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