3 GW measures average power. Google just buys the entire output of wind or solar farms, so supply is not matched with demand.[1] This is a known problem with renewables. Elon Musk claims that he can solve it with a relatively small area of batteries like the Powerpack, but he's sold less than 1 GW so far because batteries are expensive.

[1] https://environment.google/projects/announcement-100/