Depends on the years, and what you mean by faster. A 98-02 Z28 will beat a 300zx and 3000GT in top speed and acceleration, although a 3000GT VR4 will give it a good run for it's money in a 0-60 run. A 93-97 Z28 should be pretty close to those vehicles. For handling I don't know who would win, but I doubt it would be the Z28.
A regular 300zx and 3000GT aren't matches in straight line speed for a newer Z28, but a 300zxTT or 3000GT VR4 would be closer.
Submitted for the approval of the Midnight Society