Verse 3, like verse 2, contains two phrases. Those phrases are identical except that the latter is extended by a single measure (five bars long instead of four). Both reprise the (a) melody from the first and second verses, but with new (d) harmonies.
2:40-3:31 (B) Verse 3 (9 measures)
2:40 (a & d) |C G |a |C G |a | “There's a feeling...”
2:52 (a & d') |C G |a |C G |a |C G | “In my thoughts...”
This third verse cannot be called a new section because it brings back an old melody. But it also cannot be called an old section, either, because the chords are new. So it's half new, half old. This is a technique known as organic development because the music grows out of what came before, just like a seed sprouts and flowers. I analyzed the organic development of 'Good Times Bad Times' earlier. This is the same technique, but on a much grander scale.
While Verse 3 is the first time those (d) harmonies are used, they will be heard again six more times (two each in verses 4, 5, and 6):
3:31 (d) Verse 4
3:42 (d') Verse 4
4:20 (d) Verse 5
4:31 (d') Verse 5
5:08 (d) Verse 6
5:19 (d') Verse 6
Verse 4 is identical to verse 3, just with different lyrics.
3:31-3:55 (B) Verse 4 (9 measures)
3:31 (a & d) |C G |a |C G |a | “And it's whispered...”
3:42 (a & d') |C G |a |C G |a |C G | “And a new day...”
Aaron Krerowicz, pop music scholar
An informal but highly analytic study of the music of Led Zeppelin.