How does the mood and tone shift in part 3 of "The Lady of Shalott"?

In part 3, the reader is introduced to Sir Lancelot, and almost every image associated with him concerns brightness and light. The sun "came dazzling" through the leaves when he rides, and it "flam'd" upon his armor. His shield "sparkled," and his "gemmy bridle glitter'd" like stars in a "golden Galaxy." His baldric is "blazon'd," and his saddle is "thick-jewell'd" so that it "shone." His helmet and its feather "burn'd like one burning flame together," and his presence is compared to the light trailed by "bome bearded meteor" beneath the "starry clusters bright" over Shalott. His horse has "burnish'd hooves" and his brow "glow'd" in the sunlight. In short, Sir Lancelot is associated with burning and brightness over and over, and this begins to change the mood of the poem which, before, had been tragic but relatively serene. Fire and light can be beautiful, but they can also be dangerous, just as the sight of Sir Lancelot is so beautiful to the Lady of Shalott that she turns immediately to look at her window, bringing down the curse that will end her life. She is drawn to him, and she is figuratively burned by his appearance.

In addition, lines 1–4 and 6–8 of final stanza of part 3 are written in regular iambic tetrameter, though these lines in other stanzas (so far) do vary at times between iambic tetrameter and trochaic tetrameter. Suddenly, the poem shifts into a much more regular and predictably rhythmic pattern, so that it feels as though the Lady of Shalott's fate is sealed; it has become inevitable, like the regular rhythm of these lines.

What is the mood of "The Lady of Shalott"?

The lyrical poem, The Lady of Shallot's mood is created by the descriptive style that Tennyson employs in the early parts of the poem.  We learn of the landscape in great detail, until he tells us of the lady locked in the tower.

"On either side the river lie
Long fields of barley and of rye,
That clothe the wold and meet the sky;
And through the field the road run by" (Tennyson)

"Willows whiten, aspens quiver,
Little breezes dusk and shiver
Through the wave that runs for ever
By the island in the river" (Tennyson)

There is a wonder and awe that is created in the poet's description of the landscape.  The beauty of the land is contrasted to the isolation of the lady in the tower. 

She cannot enjoy this beauty, she is cursed to remain alone and cannot even look upon the world.  She sees the world through a mirror.

"And moving through a mirror clear
That hangs before her all the year,
Shadows of the world appear.
There she sees the highway near
Winding down to Camelot;" (Tennyson)

When you understand what her life is like, the mood of the poem turns somber.  Then the mood turns happy, she is content in the tower, weaving the images she sees in the mirror.

"But in her web she still delights
To weave the mirror's magic sights,
For often through the silent nights" (Tennyson)

Then the mood turns again, she falls in love with Lancelot, which makes her determined to leave the tower.
 

What is the mood of "The Lady of Shalott"?

At first, the mood of the poem is pensive (thoughtful). The Lady in her tower is only allowed to gaze upon the world through a mirror, and she is isolated. The mood then shifts to one of love or infatuation. As she looks upon Lancelot, she falls in love and literally shatters her reality. Lastly, the mood shifts once more, perhaps, to one of self-awareness as the Lady leaves the tower to float to Camelot which will signal her death. However, all is not melancholy (sad); when the Lady chooses to take her chance upon the water, she, ironically, has taken her life (or death) into her own hands and is finally deciding upon her own course of action!

What mood does the second stanza's description of the island in "The Lady of Shalott" create?

In the second stanza of "The Lady of Shalott," Tennyson creates a mood of contrasts concerning the island where the lady lives.

Initially, the island appears to be a place of beauty and quiet:

Willows whiten, aspens shiver.

The sunbeam showers break and quiver

In the stream that runneth ever

By the island in the river

Flowing down to Camelot.

These images of light and the stream flowing steadily suggest pastoral idealism. This is a place separated from the bustle of civilization. However, in the second half of the stanza, this pastoral image is complicated when the poet describes the lady's island tower.

Four gray walls, and four gray towers

Overlook a space of flowers,

And the silent isle imbowers

The Lady of Shalott.

The gray walls are a great contrast with the beauty of the trees, sunlight, and stream. They are distinctly man-made. They also suggest a prison even before the reader is made aware of the lady's discontent within the walls. The flowers are another good contrast, suggestive of the lady's beauty and delicacy, and this connection is strengthened when the poet describes the lady as being "imbowered" within the tower. That is, the tower has become a garden enclosure, and she is the flower trapped within it.

Overall, the mood here is both lovely and slightly foreboding. The island and the lady living there are beautiful, but there is undoubtedly something tragic about both. The lady is presented as a prisoner, even if she is not strictly called as much this early in the poem.