Can I say "three millennia" instead of "3000 years"?
Although the meaning of "three millennia" and "3000 years" is the same, it sounds a bit wordy and awkward here. "3000 years" sounds most natural in this context, however "millennium" or "X millennia" might sound better in other contexts (for example, when wanting to emphasize certain achievements or historical events). Ex: It was the first earthquake of this millennium. Ex: This is the most significant scientific discovery in over two millennia.