The poem of the third song integrates the world of nature and the world of love, that were separated in the first two poems. Nature has to bring over the greetings of the I-figure and has to tell her that he is full of tears.
The third song formally starts in C major, but in practice never reaches the tonic. It is just a transformation process from the D-chord at the end of song 2 through G, G7, c minor into As major. The contrast in tempo with the Adagio at the end of song 2 is large. The piano repeats the motive of the second song in a slower speed crossing over the measure bars and accompanied already with the 1/8 chords separated by rests in the left hand as an introduction of the light characteristic of the melody of the third song.
The theme of the third song is derived from the second motive of the first song. If you pick the tones C-B-As-C-Es-As from the voice in m. 5-7 you just have the transposed melody of the second motive (m. 3-5). The melody is already introduced in the piano in m. 2 and 3 through the first notes of each triplet in the right hand and is also hided in the chords in the left hand. The melody is accompanied in a stable As harmony, colored with diminished (m. 3) and Bes7 (m. 4) chords on the second beats. In m. 8 the first half of the theme is finished with the motive in the bass of m. 45 of song 2 (Es-F-D in song 2, m. 45 becomes F-G-ES). The lightness of the melody is consistent with the text of the "Leichte Segler in den Höhen", where the downwards figure in bass and voice in combination of the murmuring sound of the right hand piano on "und du Bächlein" represent the text perfectly.
The end of the brook is an Es chord, and the start of the second half of the theme that goes back from the dominant to the tonic. The rising line of the melody of the voice, the chromatic steps in m. 10 and 11 increase tension, and the harmony ends in an incomplete diminished chord on the last beat of m. 10. The start of the motive resembles by the way m. 4 of the first song, i.e. another aspect of the second theme of the first song. Then the melody goes downwards to the accentuated word "tausend" in m. 12.
The piano part of m. 11 and 12 is repeated in the piano interlude in m. 13 and 14, except for the last two bars of m. 14. The second presentation of the theme in m. 15 till 22 is the same for the voice, except for the use of full 1/4 notes instead of 1/8 notes with a rest. The rhythm in m. 17 till 20 is 3/8-1/8. This creates a little bit more unrest, although there is no immediate reason for this in the text. Beethoven may have done this because of the text in the next verse, where he asks the birds to tell about his pain.
The third verse is in as minor, as already introduced in m. 24 of the piano interlude by the adjustment of the melody with d's in it. The full chords, and the large rests in the first two measures, of the piano, reinforce the dramatic and painful effect. The complex chords in m. 30, including the diminished chord on the last beat, reinforce the effect, as do the chromatic movements and the ritardando. The piano interlude repeats in m. 33 shortly the effect of m. 31/32, including the ritardando, but then retakes a more optimistic feeling when the text mentions the "Stille Weste". The full chords in a 1/8 rhythm, just following the minor voice line, with a more optimistic V (Es) in m. 39 that transforms on the word "vergehen" in diminished chords in the second half of m. 40 and the second half of m. 41. Recognize also the effect on "Seufzer" in m. 39, comparable with the effect of "Seufzer" in m. 26 of song 1.
The voice line of the fifth verse (m. 45-52) is the same as the fourth verse. But the piano retakes the triplet accompaniment, now both in left and right hand. The triplets are stable during m. 45-46, except for the mirror of the voice line in the right hand during m. 45. It is very suggestive for the whispering in the text. In m. 47 the murmuring motive of the Bach of m. 8 is repeated. In m. 51, on the word "Thränen" the harmonies are again full of color and a ritardando is introduced again. In m. 53 on the first beat a diminished chord is used, that transforms to Es on the second "Zahl", suggesting a transformation towards the next song. Recognize also that the transformation is accomplished through continues Es in the voice, so it is the only transformation between two songs without the voice having some break.
If we summarize this song, it is obvious that Beethoven interpreted the text. The lightness of nature is represented in a subtle way, where the sad message is represented in minor with colored harmonies. Images like "Seufzer", "Flüstern", "Thränen" and the murmuring "Bächlein" and "Wogen" are made visible in the music. The motive on "blaue Nebelland" in song 1 is visible both in the first half of the theme and the second half of the theme, although it is hided to a large extent in a more complex pattern.