I am installing solar panels on the roof and a few of the existing plumbing vent stacks are in the way. Is it within California Plumbing Code to put a 90 degree angle 3 inches above the roof and reroute the vents to pop up someplace else?

Hmmm... why not run the vents inside and then pop out of roof in new location?

As Mark said: reroute it inside the attic with 90 bend and up-slope of 1/4" per foot. Penetrate roof in the area where there are no solar panels

Milo