I think it's 3, as long as the cover has a background (the background should be refreshing and not too flowery, otherwise it will affect the effect). Because the cover will be monotonous without background, putting some background related to the theme will make the overall effect better. But if you cover it all, it will make your creative plan look a little small compared with the cover. So I think it's option 3.

I hope the answer can help you, thank you! )