I have 3 parallel runs of 400 kcmil in 3.5" conduits for a 1000 amp service. What size wire do I need for the ground bushings?

I'm not trying to be a horse's butt here but you need to be using the NEC tables to determine the correct size.

This is not a DIY project by any means, please contact a licensed electrician to have this work completed safely and properly.