QIf I have 3 Natura cards on my field or in my EX area, and I play this follower, do I look at the top 3 cards of my deck with its Fanfare ability?

AIt depends. If you put a Naterran Great Tree token onto your field or into your EX area with this follower's Fanfare ability, you will have at least 5 Natura cards on your field and/or in your EX area, so you would look at the top 3 cards of your deck. However, if you choose not to put a Naterran Great Tree onto your field or into your EX area, you will only have 4 Natura cards on your field and/or in your EX area, so you wouldn't look at the top 3 cards of your deck.

QIf my field is full, do I have to put a Naterran Great Tree token into my EX area instead?

ANo, you don't.