The poem "Three years she grew" (sometimes titled "The Education of Nature") is one of Wordsworth's "Lucy poems." This is the poem that describes how Nature educates Lucy.
The poem opens by noting how Lucy "grew in sun and shower." That is to say she is like a flower, given nutrients by the sun and water. The personified Nature claims that she will "take" the child (Lucy) and make her a lady. This is to be understood as Nature adopting Lucy.
In the second stanza, Nature claims that she will teach Lucy when to follow "law" and when to reject or accept "impulse." Nature will be "an overseeing power" for Lucy. Wordsworth thinks of Nature almost like a conscious entity, a guardian angel or God. People who are more in tune with Nature will therefore be more in tune with the world and with themselves. So, if Lucy is in tune to the guiding wisdom of Nature, she will have a better understanding of herself and the world. The "law" could mean a variety of things: legal law, physics, ethical law, love, etc.
In the third stanza, Lucy is no longer described as a fixed flower. Rather, she will learn to be as free as a fawn. Lucy will learn to silence and calm. This will better enable her to be in tune with nature.
In the fourth stanza, Nature adds that Lucy will receive "Grace" from the sky, earth, and storms. Here, the natural world is described like God. The beauty of nature (in sky, earth, and storm) will enlighten Lucy. "Grace" could mean the favor of God or grace as elegance and refinement.
In the fifth stanza, Nature adds that the subtle beauty of things like stars and streams ("rivulets") will make Lucy beautiful. This idea of internal beauty transforming to external beauty continues in the next stanza. These "vital feelings of delight" will "rear her form to stately height." These "feelings" will change her "form" into something beautiful. Noting her "virgin bosom" could indicate that Lucy is becoming a young adult and thus developing into a beautiful young woman.
In the last stanza, we learn that Lucy has died. At this point, Nature is done speaking. And now the poem's speaker says that all he has left is the memory of her and the quiet natural world that she was so significantly linked to.