In fact, in metrical poetry, it is not only the third sentence that does not rhyme, but also has a oblique tone. Except for the first sentence, all singular sentences are like this.
In other words, in all ancient poetry, whether it is ancient poetry or modern poetry, basically all singular sentences (except the first sentence) do not rhyme, and use the opposite rhyme to the whole poem. If an even-numbered sentence ends with a flat tone, the odd-numbered sentence ends with a flat tone; if an even-numbered sentence ends with a flat tone (ancient poetry), the odd-numbered sentence ends with a flat tone.
Why? Because of the sentence break. There are two lines in a couplet, and the sentence is finished, but the meaning of the poem is not finished. Today we finished one sentence, but the meaning is not finished. What should we do? Let's put a comma.
But what should the ancients do if they didn’t have punctuation marks? Then the difference can only be made when reciting. If you rhyme with a flat tone, then the flat tone represents the end of the poem. When you are halfway through the sentence, of course you can't rhyme with a flat tone to give others the wrong impression. Here we use the oblique tone characters without rhyming. In fact, it tells the readers of the poem that there is still half a sentence after it. .
On the other hand, the poem is in oblique rhyme, so here the flat rhyme is used to break the sentence, also to remind readers.
In modern metrical poems, if the third sentence uses oblique characters in the same rhyme part as the even-numbered sentences, we call it "rhyme clashing"; if flat tones are used, we call it "stepping on the feet" . These are all pitfalls that need to be avoided in metrical poetry.
In fact, whether it is metrical poetry or ancient style, we must try to avoid this situation.
Another point is that the recitation of verses requires cadence, ups and downs. If the rhyme is flat all the way, will it feel lazy and endless when reading? It has the feeling of limerick mentioned in the question.