In Hebrew numerology, you probably know that three and seven are important. This make multiples (3 x 7 = 21) stand out as well.
So I find it amusing that in the Garden, the word "eat" is stated 21 times in the Hebrew in Genesis 1 and 2.
And "eating" is tied to the greenery.
Interestingly, similar to the way the animals in Genesis 1 are described in triplicate (livestock, creeping things, wild animals), we are shown three kinds of plants: grass, herbs, and trees.
In Genesis 6 and 7, the text is specific: God brings all three types of animals. But there's no mention of the three types of plants. It is only hinted at in this phrase: מִכָּל־מַאֲכָל (mi-kol ma'akhal). This phrase means "of every food."
You are to take every kind of food that is to be eaten and store it away as food for you and for them.
Genesis 6:21 (NIV)
I used to wonder why humans and animals were vegetarian before the flood. Perhaps it's linked to atonement: the Ark is a grand parable of humanity being preserved.
The three animals are a metaphor for all people, and people should not devour one another. God has provided every food we need.